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Contact process

In the first step, sulfur is burned to produce sulfur dioxide.

The sulfur dioxide is oxidized to sulfur trioxide by oxygen in the presence of a vanadium(V) oxide catalyst. This reaction is reversible and the formation of the sulfur trioxide is exothermic.

The sulfur trioxide is absorbed into 97–98% H SO to form oleum (H S O), also known as fuming sulfuric acid or pyrosulphuric acid. The oleum is then diluted with water to form concentrated sulfuric acid.

Directly dissolving SO in water, called the " wet sulfuric acid process ", is rarely practiced because the reaction is extremely exothermic, resulting in a hot aerosol of sulfuric acid that requires condensation and separation.