any of a class of chemical compounds in which the atomic group −O−O−H replaces the −O−H group of an oxy acid (a compound in which a hydrogen atom is attached to an oxygen atom by a covalent bond that is easily broken, producing an anion and a hydrogen ion). Examples of peroxy acids are peroxyacetic acid (CH₃CO−OOH, related to acetic acid, CH₃CO−OH) and peroxysulfuric acid (HOSO₂−OOH, or H₂SO₅, analogous to sulfuric acid, HOSO₂−OH, or H₂SO₄).

Peroxy acids usually are prepared by reaction of the oxy acid with hydrogen peroxide; small amounts of sulfuric or other strong acids often are used to accelerate the reaction of weak oxy acids. The peroxy acids are used primarily as oxidizing agents; they readily add oxygen to alkenes to give epoxides and are used to convert ketones to esters and amines to nitro compounds, amine oxides, or nitroso compounds.