OleumOil (Latin oleum= "oil"), or fuming sulfuric acid, is a solution of various compositions of sulphur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid).
Oleum is produced in the contact process, where sulfur is oxidized to sulfur trioxide which is subsequently dissolved in concentrated sulfuric acid. Sulfuric acid itself is regenerated by dilution of part of the oleum.
Commercially available grades with us are 65% 23% which are colorless and very fumic.