Oleum (Latin oleum, meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Oleum is identified by the CAS number 8014-95-7 (EC/List number: 616-954-1 ; ECHA InfoCard: 100.116.872).

Oleums can be described by the formula ySO₃·H₂O where y is the total molar mass of sulfur trioxide content. The value of y can be varied, to include different oleums. They can also be described by the formula H₂SO₄·xSO₃ where x is now defined as the molar free sulfur trioxide content. Oleum is generally assessed according to the free SO₃ content by mass. It can also be expressed as a percentage of sulfuric acid strength; for oleum concentrations, that would be over 100%. For example, 10% oleum can also be expressed as H₂SO₄·0.13611SO₃, 1.13611SO₃·H₂O or 102.25% sulfuric acid. The conversion between % acid and % oleum is: % acid = 100 + 18/80 × % oleum

For x = 1 and y = 2 the empirical formula H₂S₂O₇ for disulfuric (pyrosulfuric) acid is obtained. Pure disulfuric acid is a solid at room temperature, melting at 36 °C and rarely used either in the laboratory or industrial processes. Although recent research indicates that pure disulfuric acid has never been isolated yet.