# Oleum

**Oleum** (CAS 8014-95-7), also called fuming sulfuric acid, is a solution of sulfur trioxide (SO₃) dissolved in concentrated sulfuric acid (H₂SO₄)<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>. It is a heavy, oily, colorless to slightly colored liquid that fumes strongly in moist air, reacts violently with water, and is extremely corrosive<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup><sup> • </sup><sup>[2](https://ncbi.nlm.nih.gov/books/NBK598202/table/ch3.tab2/)</sup>. Oleum is an intermediate in sulfuric acid manufacture and a major industrial sulfonating and sulfating agent<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>.

| Key fact | Detail |
|---|---|
| Composition | Solution of free sulfur trioxide in 100% sulfuric acid; empirical formula H₂SO₄·xSO₃, computed formula H₂O₇S₂, molecular weight 178.13 g/mol<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup><sup> • </sup><sup>[3](https://www.fishersci.com/us/en/browse/cas/8014-95-7)</sup> |
| Strength notation | Expressed as weight percent free dissolved SO₃, or as the equivalent percentage of 100% sulfuric acid (values above 100%)<sup>[4](https://www.ncbi.nlm.nih.gov/books/NBK598202/)</sup> |
| Commercial grades | Roughly 10% to 70% free SO₃ depending on supplier; grades up to about 80% free SO₃ are available<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup> |
| Production | Absorption of contact-process SO₃ in concentrated sulfuric acid in special oleum towers<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup><sup> • </sup><sup>[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1011QO6.TXT)</sup> |
| Physical behavior | Heavy, oily, nonflammable liquid; density 114.70 lb/ft³; viscosity 38 mPa·s at 25 °C for 30% SO₃ oleum; fumes in moist air<sup>[2](https://ncbi.nlm.nih.gov/books/NBK598202/table/ch3.tab2/)</sup><sup> • </sup><sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup> |
| Transport | UN 1831, DOT emergency guide 137 (sulfuric acid, fuming)<sup>[6](https://www.cdc.gov/niosh/npg/npgd0577.html)</sup> |
| Process safety | Under OSHA's Process Safety Management standard, a process containing 1,000 pounds (453.6 kg) or more of fuming sulfuric acid at 65% by weight SO₃ or greater is covered<sup>[7](https://www.osha.gov/laws-regs/standardinterpretations/1993-06-24-0)</sup> |

## Production in the contact process

Oleum is made alongside sulfuric acid in contact plants, where sulfur dioxide is catalytically oxidized to sulfur trioxide and the SO₃ is absorbed in concentrated acid<sup>[8](https://www.vaisala.com/en/chemical-industry-solutions/chemicals-allied-products/contact-process-sulfuric-acid-and-oleum)</sup>. Plants that make oleum differ from acid-only plants in having one or two additional packed towers, irrigated with oleum, ahead of the normal SO₃ absorption towers<sup>[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1011QO6.TXT)</sup>. [Sulfuric acid](https://www.edgepedia.org/sulfuric-acid) is added to these towers to maintain the desired oleum concentration<sup>[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1011QO6.TXT)</sup>.

Absorption in the oleum tower is incomplete. Because high-concentration oleums exert a high SO₃ vapor pressure, only 30% to 60% of the sulfur trioxide in the product gas is absorbed there; the remainder is captured downstream by concentrated 98% to 99% sulfuric acid<sup>[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1011QO6.TXT)</sup>. In the double contact double absorption (DCDA) configuration, gases pass through several catalytic beds with an intermediate absorber between them, and the final product oleum can be diluted with water in a dilution tank to reach the concentration wanted<sup>[8](https://www.vaisala.com/en/chemical-industry-solutions/chemicals-allied-products/contact-process-sulfuric-acid-and-oleum)</sup>.

**Strength limits and blending.** A sulfuric acid plant can produce oleum up to 40% strength using an oleum absorption tower between the converter and the final 98% acid absorber; oleums stronger than 40% free SO₃ are made by mixing sulfur trioxide with lower-concentration oleums<sup>[9](https://www3.epa.gov/airquality/ctg_act/197709_sox_epa450_2-77-019_sulfuric_acid_mist.pdf)</sup><sup> • </sup><sup>[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1011QO6.TXT)</sup>. Liquid sulfur trioxide itself is obtained by heating oleum in a steel boiler to generate SO₃ vapor, which is then condensed; the heat is conveniently supplied by hot contact gas from the associated acid plant<sup>[5](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1011QO6.TXT)</sup>.

## Strength notation and grades

The strength of oleum is indicated by the percentage of free dissolved sulfur trioxide, or as the equivalent percentage of 100% sulfuric acid<sup>[4](https://www.ncbi.nlm.nih.gov/books/NBK598202/)</sup>. A 20% oleum, for example, contains 20% free SO₃ and 80% H₂SO₄<sup>[9](https://www3.epa.gov/airquality/ctg_act/197709_sox_epa450_2-77-019_sulfuric_acid_mist.pdf)</sup>. Because the dissolved SO₃ would form additional sulfuric acid on hydration, oleum can also be stated as an apparent sulfuric acid percentage above 100%: the Report on Carcinogens lists grades with free SO₃ content from 20% to 99.9%, corresponding to sulfuric acid equivalents from 104.5% to 122.5%<sup>[10](https://www.ncbi.nlm.nih.gov/books/NBK590831/?report=printable)</sup>.

At the composition H₂SO₄·SO₃ the dissolved sulfur trioxide corresponds to disulfuric (pyrosulfuric) acid, H₂S₂O₇, and the EPA permits reference to oleum as various compositions of sulfur trioxide in sulfuric acid or, more specifically, to disulfuric acid<sup>[11](https://www.epa.gov/sites/default/files/2015-08/documents/chemtrade_refinery_services_riverton_smnsr_permit_admin_record.pdf)</sup>. Commercial grades run from 10% to 70% free SO₃ depending on supplier, with grades up to about 80% free SO₃ available<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>.

## Uses

Oleum serves as a sulfating and sulfonating agent, a dehydrating agent in nitrations, and a laboratory reagent<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>. It is used to make surfactants, dyes, nitrocellulose, explosives, and caprolactam, and it is also used in petroleum refining and as a drying agent for chlorine and nitric acid<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>. In detergent manufacture, the oleum sulfonation and sulfation route brings oleum into intimate contact with alkylbenzenes or alcohols in a reactor; the diluted sulfuric acid byproduct is drawn off and returned to an oleum manufacturer for reprocessing<sup>[12](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=30005LG5.TXT)</sup>. Oleum is also a chemical intermediate in the manufacture of linear alkylbenzene sulfonate surfactants<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>.

Because liquid sulfur trioxide is dangerous to handle, transport, and store, oleum is often used as a source of sulfur trioxide vapor, generated on site by heating high-strength oleum in a reboiler<sup>[13](https://patents.justia.com/patent/4673560)</sup>.

## Hazards and regulation

Oleum fumes strongly in moist air, is extremely hygroscopic, and reacts violently with water<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>. It is very toxic by inhalation and corrosive to metals and tissue, quickly causing severe burns<sup>[1](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)</sup>. The International Chemical Safety Card classifies it as a mixture of sulfuric acid and sulfur trioxide with fire and explosion hazards on contact with incompatible materials, and it must not contact water<sup>[14](https://www.inchem.org/documents/icsc/icsc/eics1447.htm)</sup>. Both oleum and sulfur trioxide react with water and water vapor to form sulfuric acid mists<sup>[10](https://www.ncbi.nlm.nih.gov/books/NBK590831/?report=printable)</sup>.

**Regulatory coverage.** Ordinary sulfuric acid of 93% to 98% concentration is not covered by OSHA's Process Safety Management (PSM) standard, but a workplace process containing 1,000 pounds (453.6 kg) or more of fuming sulfuric acid at 65% by weight SO₃ or greater falls under PSM<sup>[7](https://www.osha.gov/laws-regs/standardinterpretations/1993-06-24-0)</sup>.

Accidental releases can be large-scale events. In October 2008, an uncontrolled oleum release at INDSPEC Chemical Corporation in Petrolia, Pennsylvania, forced the evacuation of three surrounding towns; the U.S. Chemical Safety Board found the release occurred when a tank transfer operation was left unattended during weekend operations and an oleum storage tank overflowed, a practice not considered in process hazard analyses or operating procedures<sup>[15](https://www.csb.gov/csb-issues-final-report-on-oleum-release-from-indspec-chemical-corp-that-forced-thousands-to-evacuate-in-october-2008-report-cites-use-of-pump-power-supply-that-lacked-safeguards/?F_All=y&eId=ddf69081-cafa-449a-8cc2-33ab531688f8&eType=EmailBlastContent)</sup>.

## References

1. [Fuming sulfuric acid (Oleum), CID 24681, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/Oleum)
2. [Table 3-2, Physical and Chemical Properties of Sulfur Trioxide, Sulfuric Acid, and Oleum, Toxicological Profile for Sulfur Trioxide and Sulfuric Acid, NCBI Bookshelf](https://ncbi.nlm.nih.gov/books/NBK598202/table/ch3.tab2/)
3. [CAS RN 8014-95-7, Fisher Scientific](https://www.fishersci.com/us/en/browse/cas/8014-95-7)
4. [CHEMICAL AND PHYSICAL INFORMATION, Toxicological Profile for Sulfur Trioxide and Sulfuric Acid, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK598202/)
5. [Prevention Reference Manual, Volume 15: Control of Accidental Releases of Sulfur Trioxide, EPA](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1011QO6.TXT)
6. [Sulfuric acid, NIOSH Pocket Guide to Chemical Hazards, CDC](https://www.cdc.gov/niosh/npg/npgd0577.html)
7. [Sulfuric acid and the PSM standard, OSHA interpretation letter, June 24, 1993](https://www.osha.gov/laws-regs/standardinterpretations/1993-06-24-0)
8. [Contact Process: Sulfuric Acid and Oleum, Vaisala](https://www.vaisala.com/en/chemical-industry-solutions/chemicals-allied-products/contact-process-sulfuric-acid-and-oleum)
9. [EPA 450/2-77-019, Control of Sulfuric Acid Mist Emissions from Existing Sulfuric Acid Production Units](https://www3.epa.gov/airquality/ctg_act/197709_sox_epa450_2-77-019_sulfuric_acid_mist.pdf)
10. [Strong Inorganic Acid Mists Containing Sulfuric Acid, 15th Report on Carcinogens, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK590831/?report=printable)
11. [A. Plant #1 Process Description, Chemtrade Refinery Services Riverton permit, EPA](https://www.epa.gov/sites/default/files/2015-08/documents/chemtrade_refinery_services_riverton_smnsr_permit_admin_record.pdf)
12. [Soap and Detergent: Manufacturing Point Source Category, EPA](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=30005LG5.TXT)
13. [U.S. Patent 4,673,560, Generation of sulphur trioxide from oleum using microwave energy, Justia Patents](https://patents.justia.com/patent/4673560)
14. [ICSC 1447 - OLEUM, International Chemical Safety Cards](https://www.inchem.org/documents/icsc/icsc/eics1447.htm)
15. [CSB Issues Final Report on Oleum Release from INDSPEC Chemical Corp., U.S. Chemical Safety Board](https://www.csb.gov/csb-issues-final-report-on-oleum-release-from-indspec-chemical-corp-that-forced-thousands-to-evacuate-in-october-2008-report-cites-use-of-pump-power-supply-that-lacked-safeguards/?F_All=y&eId=ddf69081-cafa-449a-8cc2-33ab531688f8&eType=EmailBlastContent)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Elements and inorganic substances › Sulfur oxides and sulfates › Sulfur oxide substances*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license