Baeyer–Villiger oxidation converts aldehydes to carboxylic acids and ketones to esters. The reaction uses peroxy acids or peracids and is often catalyzed by acid. The reaction is named after its pioneers, Adolf von Baeyer and Victor Villiger. The reaction is achieved by a wide range of peracids such as m-chloroperoxybenzoic acid (mCPBA), perbenzoic acid (C₆H₅COOOH), peracetic acid (CH₃COOOH), hydrogen peroxide (H₂O₂), and tert-butyl hydroperoxide (t-BuOOH).

The carbonyl center is activated by protonating the carbonyl oxygen. The peracid then adds across the C=O bond to give a tetrahedral intermediate called the Criegee intermediate. During the concerted intramolecular rearrangement that follows, the carbonyl group is restored, a group migrates from carbon to oxygen, and the weak peroxide bond cleaves to give an acid (if the substrate is an aldehyde) or an ester (if the substrate is a ketone).

The migratory aptitude of different groups follows the order of –H > –CR₃ > –CHR₂ ≈ –Ph > –CRH₂ > –⁠CH₃. This makes Baeyer–Villiger oxidation regioselective. Also, the reaction retains the stereochemistry at the migrating center for asymmetrical substrates. Reactions involving cyclic ketones produce lactones.