[1] Oleum is an important intermediate in the manufacture of sulfuric acid due to its high enthalpy of hydration.

When SO3 is added to water, rather than dissolving, it tends to form a fine mist of sulfuric acid, which is difficult to manage.

Adding sulfur trioxide alters the chemical equilibrium, allowing concentration to be increased beyond 98.3%.

In addition, oleum lacks free water to attack surfaces, making it less corrosive to metals.

In Richmond, California in 1993 a significant release occurred due to overheating, causing a release of sulfur trioxide[6] that absorbed moisture from the atmosphere, creating a mist of micrometre-sized sulfuric acid particles that formed an inhalation health hazard.

A stronger reagent, oleum, is needed to introduce the second nitro group onto the aromatic ring.

Fuming nitric acid is hazardous to handle and transport, because it is extremely corrosive and volatile.

[11] Like concentrated sulfuric acid, oleum is such a strong dehydrating agent that if poured onto powdered glucose, or virtually any other sugar, it will draw the hydrogen elements of water out of the sugar in an exothermic reaction, leaving a residue of nearly pure carbon as a solid.

This carbon expands outward, hardening as a solid black substance with gas bubbles in it.