Osmium tetroxide

Athanor · official

O₄Os · 254.2 g/mol

IUPAC name: tetraoxoosmium

Also called osmic acid, EM stain, lipid stain

Reagents Inorganic

Open in Athanor

Drawn from its SMILES with RDKit

DangerFatal if swallowed · Fatal in contact with skin · Causes severe skin burns and eye damage · Fatal if inhaled

Names and identifiers

IUPAC name: tetraoxoosmium
Synonyms: osmic acid, EM stain, lipid stain
+ 38 more
Osmium tetraoxide, Osmic acid, tetraoxoosmium, Perosmic oxide, Osmium(VIII) oxide, Osmic acid anhydride, Perosmic acid anhydride, Osmium oxide (OsO4), (T-4)-, RCRA waste number P087, P40W033BGM, DTXSID5042245, CHEBI:88215, Acid, Osmic, Tetroxide, Osmium, RefChem:168681, OSMIUM OXIDE (OSO4), DTXCID3022245, 244-058-7, OsO4, Osmium oxide, (T-4)-, Osmium(VIII)oxide, Osmium Tetroxide (4% in H2O), Osmiumtetroxide, osmium(VIII)tetraoxide, RCRA waste no. P087, OSMIUM (VIII) OXIDE, osmiumtetroxid, osmiumtetraoxide, HSDB 719, Osmium oxide (OsO4) (T-4)-, Osmium tetra oxide, osmium(IV) oxide, EINECS 244-058-7, Osmium oxide (OsO4), (beta-4)-, UN2471, Tetrakis(oxido)osmium, Osmium(VIII)-tetroxide, osmium (VIII) tetraoxide
Molecular formula: O₄Os
Empirical formula: O₄Os
Molecular weight: 254.2 g/mol
Exact mass: 255.94114
CAS number: 20816-12-0
PubChem CID: 30318
Canonical SMILES: O=[Os](=O)(=O)=O
Isomeric SMILES: O=[Os](=O)(=O)=O
InChI: InChI=1S/4O.Os
InChIKey: VUVGYHUDAICLFK-UHFFFAOYSA-N
Molfile

Names, InChI and the identifiers from PubChem, retrieved 2026-10-14.

Physical properties

TemperaturePressureDensity

Appearance: Dimorphic compd with both crystalline and amorphous forms[5]
6 values
From Hazardous Substances Data Bank (HSDB), the most authoritative source here.
Value As written Source
– Osmium tetroxide appears as a colorless or yellow solid with a pungent odor of chlorine. Begins to sublime below melting point. Soluble in alcohol. Toxic by inhalation and a strong irritant to the eyes and mucous membranes. [1] CAMEO Chemicals
– COLOURLESS-TO-PALE-YELLOW SOLID IN VARIOUS FORMS WITH PUNGENT ODOUR. [2] ILO-WHO International Chemical Safety Cards (ICSCs)
– Colorless, crystalline solid or pale-yellow mass with an unpleasant, acrid, chlorine-like odor. [3] Occupational Safety and Health Administration (OSHA)
– Colorless, crystalline solid or pale-yellow mass with an unpleasant, acrid, chlorine-like odor. [Note: A liquid above 105 °F.] [4] The National Institute for Occupational Safety and Health (NIOSH)
–shown Dimorphic compd with both crystalline and amorphous forms [5] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J., Sr (Ed.). Hawley's Condensed Chemical Dictionary. 13th ed. New York, NY: John Wiley & Sons, Inc. 1997., p. 826
– Colorless, crystalline solid, or pale-yellow mass [Note: A liquid above 105 degrees F]. [5] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards & Other Databases. U.S. Department of Health & Human Services, Public Health Service, Center for Disease Control & Prevention. DHHS (NIOSH) Publication No. 2001-145 (CD-ROM) August 2001.
Odor: Unpleasant odor.[5]
Melting point: 40.6 °C[6]
5 values
5 of 5 sources agree on it (most common value).
Value As written Source
41 °C 105 °F (NIOSH, 2024) [1] CAMEO Chemicals
42 °C 42 °C [2] ILO-WHO International Chemical Safety Cards (ICSCs)
41 °C 105 °F [3] Occupational Safety and Health Administration (OSHA)
40.6 °Cshown 40.6 °C [6] PAC Chemical Database, U.S. Department of Energy
41 °C 105 °F [4] The National Institute for Occupational Safety and Health (NIOSH)
Boiling point: 131.2 °C @760 [mmHg][6]
5 values
5 of 5 sources agree on it (most common value).
Value As written Source
130 °C 266 °F at 760 mmHg (NIOSH, 2024) [1] CAMEO Chemicals
130 °C 130 °C [2] ILO-WHO International Chemical Safety Cards (ICSCs)
130 °C 266 °F [3] Occupational Safety and Health Administration (OSHA)
131.2 °Cshown 131.2 °C @760 [mm Hg] [6] PAC Chemical Database, U.S. Department of Energy
130 °C 266 °F [4] The National Institute for Occupational Safety and Health (NIOSH)
Density: 5.10[4]
5 values
4 of 5 sources agree on it (most common value).
Value As written Source
– 5.1 (NIOSH, 2024) - Denser than water [1] CAMEO Chemicals
– Relative density (water = 1): 4.9 [2] ILO-WHO International Chemical Safety Cards (ICSCs)
– 5.1 [3] Occupational Safety and Health Administration (OSHA)
– 5.1 @25 °C [6] PAC Chemical Database, U.S. Department of Energy
–shown 5.10 [4] The National Institute for Occupational Safety and Health (NIOSH)
Solubility in water: Solubility in water, g/100ml at 25 °C: 6[2]
3 values
From ILO-WHO International Chemical Safety Cards (ICSCs), the most authoritative source here.
Value As written Source
– 6 % at 77 °F (NIOSH, 2024) [1] CAMEO Chemicals
–shown Solubility in water, g/100ml at 25 °C: 6 [2] ILO-WHO International Chemical Safety Cards (ICSCs)
– (77 °F): 6% [4] The National Institute for Occupational Safety and Health (NIOSH)
Vapor pressure: Vapor pressure, kPa at 27 °C: 1.5[2]
2 values
2 of 2 sources agree on it (most common value).
Value As written Source
11 mmHgshown Vapor pressure, kPa at 27 °C: 1.5 [2] ILO-WHO International Chemical Safety Cards (ICSCs)
11 mmHg 11 [mm Hg] @27 °C [6] PAC Chemical Database, U.S. Department of Energy
3 more values were reported but failed the checks (unreadable or out of range), so they are not shown.

Value	As written	Source
–	Osmium tetroxide appears as a colorless or yellow solid with a pungent odor of chlorine. Begins to sublime below melting point. Soluble in alcohol. Toxic by inhalation and a strong irritant to the eyes and mucous membranes.	[1] CAMEO Chemicals
–	COLOURLESS-TO-PALE-YELLOW SOLID IN VARIOUS FORMS WITH PUNGENT ODOUR.	[2] ILO-WHO International Chemical Safety Cards (ICSCs)
–	Colorless, crystalline solid or pale-yellow mass with an unpleasant, acrid, chlorine-like odor.	[3] Occupational Safety and Health Administration (OSHA)
–	Colorless, crystalline solid or pale-yellow mass with an unpleasant, acrid, chlorine-like odor. [Note: A liquid above 105 °F.]	[4] The National Institute for Occupational Safety and Health (NIOSH)
–shown	Dimorphic compd with both crystalline and amorphous forms	[5] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J., Sr (Ed.). Hawley's Condensed Chemical Dictionary. 13th ed. New York, NY: John Wiley & Sons, Inc. 1997., p. 826
–	Colorless, crystalline solid, or pale-yellow mass [Note: A liquid above 105 degrees F].	[5] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards & Other Databases. U.S. Department of Health & Human Services, Public Health Service, Center for Disease Control & Prevention. DHHS (NIOSH) Publication No. 2001-145 (CD-ROM) August 2001.

Value	As written	Source
41 °C	105 °F (NIOSH, 2024)	[1] CAMEO Chemicals
42 °C	42 °C	[2] ILO-WHO International Chemical Safety Cards (ICSCs)
41 °C	105 °F	[3] Occupational Safety and Health Administration (OSHA)
40.6 °Cshown	40.6 °C	[6] PAC Chemical Database, U.S. Department of Energy
41 °C	105 °F	[4] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
130 °C	266 °F at 760 mmHg (NIOSH, 2024)	[1] CAMEO Chemicals
130 °C	130 °C	[2] ILO-WHO International Chemical Safety Cards (ICSCs)
130 °C	266 °F	[3] Occupational Safety and Health Administration (OSHA)
131.2 °Cshown	131.2 °C @760 [mm Hg]	[6] PAC Chemical Database, U.S. Department of Energy
130 °C	266 °F	[4] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
–	5.1 (NIOSH, 2024) - Denser than water	[1] CAMEO Chemicals
–	Relative density (water = 1): 4.9	[2] ILO-WHO International Chemical Safety Cards (ICSCs)
–	5.1	[3] Occupational Safety and Health Administration (OSHA)
–	5.1 @25 °C	[6] PAC Chemical Database, U.S. Department of Energy
–shown	5.10	[4] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
–	6 % at 77 °F (NIOSH, 2024)	[1] CAMEO Chemicals
–shown	Solubility in water, g/100ml at 25 °C: 6	[2] ILO-WHO International Chemical Safety Cards (ICSCs)
–	(77 °F): 6%	[4] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
11 mmHgshown	Vapor pressure, kPa at 27 °C: 1.5	[2] ILO-WHO International Chemical Safety Cards (ICSCs)
11 mmHg	11 [mm Hg] @27 °C	[6] PAC Chemical Database, U.S. Department of Energy

Each value as its source reports it, with the source's number after it (listed under References). Where several sources report a property, the row shows the value most of them agree on, the rule the app uses; open the values to see every one.

Calculated

Heavy atoms: 5
Rings: 0
Aromatic rings: 0
Fraction sp³ carbons (Fsp3): 0
H-bond donors: 0
H-bond acceptors: 4
Rotatable bonds: 0
TPSA (Å²): 68.3
Formal charge: 0
Complexity: 138 PubChem
Stereocenters: None
Double-bond geometry: None specified
Point group: C1 of one 3D conformer with its hydrogens, by Athanor’s engine (as in the app)

Elemental analysis

O25.17%
Os74.83%

Isotope pattern

Each peak is a nominal mass with its abundance-weighted exact mass, as a percentage of the tallest, from IUPAC natural isotope abundances.

Computed from the structure with RDKit 2026.09 when this page was built, beside PubChem's own computed values from its record; where the two differ, both are shown.

Predicted

log P: -0.48 Crippen (Wildman and Crippen 1999)
log S (water solubility): -1.12 ESOL (Delaney 2004), log mol/L
Synthetic accessibility: 4.69 SA score, 1 easy to 10 hard (Ertl and Schuffenhauer 2009)

Drug-likeness: Lipinski’s rule of five

Molecular weight ≤ 500: 254.23
log P ≤ 5 (Crippen): -0.48
H-bond donors ≤ 5: 0
H-bond acceptors ≤ 10: 4

Passes all four rules.

Predictions, not measurements; each names its method. A measured log P, where a source reports one, is under Physical properties.

Safety

Danger

Harmonized EU classification (CLP Annex VI). Source [7].

Corrosive
Acute toxicity

H300Fatal if swallowed
H310Fatal in contact with skin
H314Causes severe skin burns and eye damage
H330Fatal if inhaled

Precautionary statements: P260, P262, P264, P270, P271, P280, P284, P301+P316, P301+P330+P331, P302+P352, P302+P361+P354, P304+P340, P305+P354+P338, P316, P320, P321, P330, P361+P364, P363, P403+P233, P405, P501

View all classifications (2)

[7] Regulation (EC) No 1272/2008 of the European Parliament and of the Council · DangerH300, H310, H314, H330
[8] European Chemicals Agency (ECHA) · 58 reports · DangerH300, H310, H314, H330, H334

Exposure limits

PEL-TWA (8-Hour Time Weighted Average): 0.002 mg/m³[3]
PEL: TWA 0.002 mg/m3 See Appendix G[4]
REL-TWA (Time Weighted Average): 0.0002 ppm (0.002 mg/m³)[3]
REL-STEL (Short Term Exposure Limit): 0.0006 ppm (0.006 mg/m³)[3]
REL: TWA 0.002 mg/m3 (0.0002 ppm) ST 0.006 mg/m3 (0.0006 ppm)[4]
TLV: (as Os): 0.0002 ppm as TWA; 0.0006 ppm as STEL[2]

First aid

Excerpt from NIOSH Pocket Guide for Osmium tetroxide: Eye: Irrigate immediately - If this chemical contacts the eyes, immediately wash (irrigate) the eyes with large amounts of water, occasionally lifting the lower and upper lids. Get medical attention immediately. Skin: SOAP WASH immediately - If this chemical contacts the skin, immediately wash the contaminated skin with soap and water.[1]
General First Aid: · Call 911 or emergency medical service. · Ensure that medical personnel are aware of the material(s) involved, take precautions to protect themselves and avoid contamination. · Move victim to fresh air if it can be done safely. · Administer oxygen if breathing is difficult. · If victim is not breathing: -- DO NOT perform mouth-to-mouth resuscitation; the victim may have ingested or inhaled the substance. -- If equipped and pulse detected, wash face and mouth, then give artificial respiration using a proper respiratory medical device (bag-valve mask, pocket mask equipped…[9]
(General first aid procedures) Eye: Irrigate immediately - If this chemical contacts the eyes, immediately wash (irrigate) the eyes with large amounts of water, occasionally lifting the lower and upper lids. Get medical attention immediately. Skin: Soap wash immediately - If this chemical contacts the skin, immediately wash the contaminated skin with soap and water.[4]
Fresh air, rest. Half-upright position. Artificial respiration may be needed. Refer for medical attention.[2]

Storage

Separated from combustible substances, reducing agents and food and feedstuffs. Ventilation along the floor.[2]
Separated from combustible and reducing substances, food and feedstuffs. Ventilation along the floor.[5]
Because of its high acute toxicity, osmium tetroxide should be handled in the laboratory using the "basic prudent practices" of Chapter 5.C, supplemented by the additional precautions for work with compounds of high toxicity (Chapter 5.D). In particular, all work with osmium tetroxide should be conducted in a fume hood to prevent exposure by inhalation, and splash goggles and impermeable gloves should be worn at all times to prevent eye and skin contact. Osmium tetroxide as solid or solutions should be stored in tightly sealed containers, and these should be placed in secondary containers.[5]

Incompatibilities

Contact with hydrochloric acid will cause formation of poisonous chlorine gas. Contact with easily oxidized organic materials may cause fires and explosions.[5]
Hydrochloric acid, easily oxidized organic materials [Note: Begins to sublime below BP. Contact with with other materials may cause fire].[5]
Explodes on contact with 1-methylimidazole.[5]
Caution: Osmium tetroxide, OsO4, a volatile, extremely poisonous substance, is formed from almost any osmium compound under acid conditions in the presence of air.[5]

Safety data sheets on the supplier’s page: RR Scientific, Thermo Fisher Scientific, Clinivex.

Always read the supplier’s safety data sheet before working with it.

Spectra

Experimental spectra held by these sources; each link opens the source's own record.

Literature

Reading PubChem, OpenAlex and GSRS…

Bioactivity

Reading ChEMBL…

References

[1]CAMEO Chemicals · Public domain (NOAA) · via PubChem · retrieved 2026-10-14
[2]ILO-WHO International Chemical Safety Cards (ICSCs) · CC BY 4.0 · via PubChem · retrieved 2026-10-14
[3]Occupational Safety and Health Administration (OSHA) · Public domain (US DOL) · via PubChem · retrieved 2026-10-14
[4]The National Institute for Occupational Safety and Health (NIOSH) · Public domain (US CDC) · via PubChem · retrieved 2026-10-14
[5]Hazardous Substances Data Bank (HSDB) · Public domain (US NLM) · via PubChem · retrieved 2026-10-14
[6]PAC Chemical Database, U.S. Department of Energy · Public domain (US DOE) · via PubChem · retrieved 2026-10-14
[7]Regulation (EC) No 1272/2008 of the European Parliament and of the Council · EU reuse policy, CC BY 4.0 · via PubChem · retrieved 2026-10-14
[8]European Chemicals Agency (ECHA) · ECHA legal notice: reuse with acknowledgement · via PubChem · retrieved 2026-10-14
[9]Emergency Response Guidebook (ERG) · Public domain (US DOT) · via PubChem · retrieved 2026-10-14
[10]SpectraBase · Linked only · via PubChem · retrieved 2026-10-14
·Calculated values: computed by Athanor with RDKit 2026.09 from the structure.
·Literature on PubChem

Data last updated 2026-10-14.