Sulfur trioxide

Athanor · official

O₃S · 80.07 g/mol

Also called sulfuric anhydride

Inorganic

Open in Athanor

Drawn from its SMILES with RDKit

DangerCauses severe skin burns and eye damage · Causes serious eye damage · Fatal if inhaled · May cause respiratory irritation · Suspected of causing cancer · Toxic to aquatic life with long lasting effects

Names and identifiers

IUPAC name: sulfur trioxide
Synonyms: sulfuric anhydride, Sulphur trioxide, Sulfur oxide (SO3)
+ 19 more
HH2O7V4LYD, DTXSID1029673, CHEBI:29384, Sulfan, Sulfuric oxide, RefChem:6276, Sulfur trioxide, stabilized, DTXCID409673, 231-197-3, Sulfuric anhydride, Sulfite radical anion, Sulfur trioxide, contains stabilizer, >=99%, NISSO SULFAN, Trioxo-lambda~6~-sulfane, SCHEMBL6713, SULFUR TRIOXIDE [MI], [SO3], NS00081874, Q242715
Molecular formula: O₃S
Empirical formula: O₃S
Molecular weight: 80.07 g/mol
Exact mass: 79.95682
CAS number: 7446-11-9
PubChem CID: 24682
Canonical SMILES: O=S(=O)=O
Isomeric SMILES: O=S(=O)=O
InChI: InChI=1S/O3S/c1-4(2)3
InChIKey: AKEJUJNQAAGONA-UHFFFAOYSA-N
Molfile

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

Physical properties

TemperaturePressureDensity

Appearance: Pure sulfur trioxide is a colorless liquid that fumes in air at room temperature and pressure.[3]
3 values
From Hazardous Substances Data Bank (HSDB), the most authoritative source here.
Value As written Source
– Sulfur trioxide, is a colorless to white crystalline solid which will fume in air. Often shipped with inhibitor to prevent polymerization. It reacts violently with water to form sulfuric acid with the release of heat. It is corrosive to metals and tissue. It causes eye and skin burns. Ingestion causes severe burns of mouth esophagus and stomach. The vapor is very toxic by inhalation. It is a fire risk when in contact with organic materials such as wood, cotton, fiberboard, etc. [1] CAMEO Chemicals
– HYGROSCOPIC LIQUID OR SOLID IN VARIOUS FORMS. . [2] ILO-WHO International Chemical Safety Cards (ICSCs)
–shown Pure sulfur trioxide is a colorless liquid that fumes in air at room temperature and pressure. [3] Hazardous Substances Data Bank (HSDB)cites Muller TL; Kirk-Othmer Encyclopedia of Chemical Technology. (2005). NY, NY: John Wiley & Sons; Sulfuric Acid and Sulfur Trioxide. Online Posting Date: May 19, 2006.
Melting point: 16.8 °C (62.2 °F) Gamma form (EPA, 1998)[1]
3 values
2 of 2 sources agree on it (most common value).
Value As written Source
32.5 °C 90.5 °F Beta form [1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
16.8 °Cshown 62.2 °F Gamma form (EPA, 1998) [1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
17 °C 17 °C [2] ILO-WHO International Chemical Safety Cards (ICSCs)
1 more value was reported but failed the checks (unreadable or out of range), so it is not shown.
Boiling point: 44.8 °C[2]
2 values
2 of 2 sources agree on it (most common value).
Value As written Source
45 °C 113 °F at 760 mmHg all forms (EPA, 1998) [1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
44.8 °Cshown 44.8 °C [2] ILO-WHO International Chemical Safety Cards (ICSCs)
Density: 1.92 at 20 °C (68 °F) Gamma form (liquid) (EPA, 1998) - Denser than water[1]
2 values
2 of 2 sources agree on it (most common value).
Value As written Source
–shown 1.92 at 68 °F Gamma form (liquid) (EPA, 1998) - Denser than water [1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
– Relative density (water = 1): 1.9 [2] ILO-WHO International Chemical Safety Cards (ICSCs)
1 more value was reported but failed the checks (unreadable or out of range), so it is not shown.
Solubility in water: Solubility in water: violent reaction[2]
2 values
From ILO-WHO International Chemical Safety Cards (ICSCs), the most authoritative source here.
Value As written Source
– Slowly very sol in aqueous, and sol in 100 parts water at room temp [3] Hazardous Substances Data Bank (HSDB)cites Dean, J.A. Handbook of Organic Chemistry. New York, NY: McGraw-Hill Book Co., 1987., p. 2-29
–shown Solubility in water: violent reaction [2] ILO-WHO International Chemical Safety Cards (ICSCs)
Vapor pressure: 433 mmHg at 25 °C (77 °F) Gamma form (EPA, 1998)[1]
4 values
2 of 2 sources agree on it (most common value).
Value As written Source
73 mmHg 73 mmHg at 77 °F Alpha form [1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
344 mmHg 344 mmHg at 77 °F Beta form [1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
433 mmHgshown 433 mmHg at 77 °F Gamma form (EPA, 1998) [1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
430 mmHg Vapor pressure, kPa at 25 °C: 57.7 [2] ILO-WHO International Chemical Safety Cards (ICSCs)

Value	As written	Source
–	Sulfur trioxide, is a colorless to white crystalline solid which will fume in air. Often shipped with inhibitor to prevent polymerization. It reacts violently with water to form sulfuric acid with the release of heat. It is corrosive to metals and tissue. It causes eye and skin burns. Ingestion causes severe burns of mouth esophagus and stomach. The vapor is very toxic by inhalation. It is a fire risk when in contact with organic materials such as wood, cotton, fiberboard, etc.	[1] CAMEO Chemicals
–	HYGROSCOPIC LIQUID OR SOLID IN VARIOUS FORMS. .	[2] ILO-WHO International Chemical Safety Cards (ICSCs)
–shown	Pure sulfur trioxide is a colorless liquid that fumes in air at room temperature and pressure.	[3] Hazardous Substances Data Bank (HSDB)cites Muller TL; Kirk-Othmer Encyclopedia of Chemical Technology. (2005). NY, NY: John Wiley & Sons; Sulfuric Acid and Sulfur Trioxide. Online Posting Date: May 19, 2006.

Value	As written	Source
32.5 °C	90.5 °F Beta form	[1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
16.8 °Cshown	62.2 °F Gamma form (EPA, 1998)	[1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
17 °C	17 °C	[2] ILO-WHO International Chemical Safety Cards (ICSCs)

Value	As written	Source
45 °C	113 °F at 760 mmHg all forms (EPA, 1998)	[1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
44.8 °Cshown	44.8 °C	[2] ILO-WHO International Chemical Safety Cards (ICSCs)

Value	As written	Source
–shown	1.92 at 68 °F Gamma form (liquid) (EPA, 1998) - Denser than water	[1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
–	Relative density (water = 1): 1.9	[2] ILO-WHO International Chemical Safety Cards (ICSCs)

Value	As written	Source
–	Slowly very sol in aqueous, and sol in 100 parts water at room temp	[3] Hazardous Substances Data Bank (HSDB)cites Dean, J.A. Handbook of Organic Chemistry. New York, NY: McGraw-Hill Book Co., 1987., p. 2-29
–shown	Solubility in water: violent reaction	[2] ILO-WHO International Chemical Safety Cards (ICSCs)

Value	As written	Source
73 mmHg	73 mmHg at 77 °F Alpha form	[1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
344 mmHg	344 mmHg at 77 °F Beta form	[1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
433 mmHgshown	433 mmHg at 77 °F Gamma form (EPA, 1998)	[1] CAMEO Chemicalscites U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
430 mmHg	Vapor pressure, kPa at 25 °C: 57.7	[2] ILO-WHO International Chemical Safety Cards (ICSCs)

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: 4
Rings: 0
Aromatic rings: 0
Fraction sp³ carbons (Fsp3): 0
H-bond donors: 0
H-bond acceptors: 3
Rotatable bonds: 0
TPSA (Å²): 51.2 computed here · PubChem 52.2
Formal charge: 0
XLogP3: -0.5 PubChem
Complexity: 61 PubChem
Stereocenters: None
Double-bond geometry: None specified
Point group: C3v of one 3D conformer with its hydrogens, by Athanor’s engine (as in the app)

Elemental analysis

O59.95%
S40.05%

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: -1 Crippen (Wildman and Crippen 1999)
log S (water solubility): 0.3 ESOL (Delaney 2004), log mol/L
Synthetic accessibility: 3.05 SA score, 1 easy to 10 hard (Ertl and Schuffenhauer 2009)

Drug-likeness: Lipinski’s rule of five

Molecular weight ≤ 500: 80.06
log P ≤ 5 (Crippen): -1
H-bond donors ≤ 5: 0
H-bond acceptors ≤ 10: 3

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

Most widely reported: 503 company reports to ECHA. Source [4].

Corrosive
Acute toxicity
Harmful or irritant
Health hazard
Environmental hazard

H314Causes severe skin burns and eye damage (85.1% of reports)
H318Causes serious eye damage (49.3% of reports)
H330Fatal if inhaled (16.1% of reports)
H335May cause respiratory irritation (79.9% of reports)
H351Suspected of causing cancer (11.3% of reports)
H411Toxic to aquatic life with long lasting effects (11.3% of reports)

Precautionary statements: P203, P260, P261, P264, P264+P265, P271, P273, P280, P284, P301+P330+P331, P302+P361+P354, P304+P340, P305+P354+P338, P316, P317, P318, P319, P320, P321, P363, P391, P403+P233, P405, P501

View all classifications (2)

[4] European Chemicals Agency (ECHA)This chemical does not meet GHS hazard criteria for 14.9% (75 of 503) of reports.
[4] European Chemicals Agency (ECHA) · 503 reports · DangerH314, H318, H330, H335, H351, H411

First aid

Warning: Sulfur trioxide is extremely corrosive. Caution is advised. Signs and Symptoms of Sulfur Trioxide Exposure: Signs and symptoms of acute ingestion of sulfur trioxide may be severe and include salivation, intense thirst, difficulty in swallowing, chills, pain, and shock. Oral, esophageal, and stomach burns are common. Vomitus generally has a coffee-ground appearance. The potential for circulatory collapse is high following ingestion of sulfur trioxide.[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…[5]
Fresh air, rest. Half-upright position. Artificial respiration may be needed. Refer immediately for medical attention.[2]
Wear protective gloves when administering first aid. First rinse with plenty of water for at least 15 minutes, then remove contaminated clothes and rinse again. Refer immediately for medical attention .[2]

Storage

Refer to the manufacturer's instructions for proper storage conditions. Dry. Well closed. Store only in original packaging. Separated from food and feedstuffs and incompatible materials. See Chemical Dangers. Store only if stabilized.[2]
(when) stabilized, must be kept warm to liquefy, but under 40 °C.[3]

Incompatibilities

The substance is a strong oxidant and reacts violently with combustible and reducing materials and organic compounds causing fire and explosion hazard. Reacts violently with water and moist air to produce sulfuric acid. The solution in water is a strong acid, it reacts violently with bases and is corrosive metals forming flammable/explosive gas (hydrogen).[3]
On exposure to air, it absorbs moisture rapidly, emitting dense white fumes. Combines with water with explosive violence ... Forming sulfuric acid.[The Merck Index, Fourteenth Edition (2006)[3]
Dissolution of sulfur trioxide in ... (dimethyl) sulfoxide is very exothermic and must be done slowly with cooling to avoid decomp.[3]
... The 1:1 addition complex (dioxane & sulfur trioxide) sometimes decomp violently on storing at ambient temp ...[3]

Safety data sheets on the supplier’s page: Sigma-Aldrich, Smolecule, BenchChem.

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

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]Hazardous Substances Data Bank (HSDB) · Public domain (US NLM) · via PubChem · retrieved 2026-10-14
[4]European Chemicals Agency (ECHA) · ECHA legal notice: reuse with acknowledgement · via PubChem · retrieved 2026-10-14
[5]Emergency Response Guidebook (ERG) · Public domain (US DOT) · 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.