Oleum, also known as fuming sulfuric acid, is a highly concentrated solution of sulfuric acid containing sulfur trioxide (SO₃).

The production process takes place via contact oxidation, whereby sulfur dioxide (SO2) is catalytically oxidized to sulfur trioxide (SO3).

In such plants, this process can be carried out using the C&CS #1394 platinum catalyst for SO₂ oxidation.

The generated SO₃ is then absorbed in concentrated sulfuric acid, producing oleum.

Due to its high SO₃ content, oleum exhibits strongly hygroscopic and corrosive behavior, which places special demands on equipment materials, process control and safety measures.

Oleum serves as an intermediate in sulfuric acid chemistry and is a starting material for numerous industrial applications. Sulfuric acid production

The concentration and purity of oleum can be precisely adjusted by controlling SO₂ oxidation and absorption.