The oxidation of acetaldehyde in glacial acetic acid catalyzed by manganese, copper and cobalt acetate has been studied at room temperature. Analyses of the products have been carried out, and it is shown that the reaction proceeds in two well-defined stages: ( a ) the oxidation of the aldehyde to peracetic acid; ( b ) the reaction of peracetic acid with acetal­dehyde to give a second peroxide, X. The metal-salt-catalyzed decomposition of both peroxides has been determined. Kinetic measurements show that the oxidation with each of the metal acetates follows a similar course and conforms to the general kinetic relationship dO 2 /d t = k [acetaldehyde]3/2 [catalyst]1/2. These measurements, together with other observations, indicate that the initiating process is an electron-transfer reaction between the aldehyde and the higher valence state of the ion of the metal catalyst: M 3+ ( M 2+ ) + R CHO → M 2+ ( M + ) + R CO• + H + . This reaction leads to the establishment of the following chain process R CO• +O 2 → R COO•, R COOO • + R CHO → R COOOH + R CO•. The metal ion is maintained in the higher valence state by the reaction cycle R COOOH + M 2+ ( M + ) → R COO• + M 3+ ( M 2+ ) + OH - , R COOOH + M 3+ ( M 2+ ) → R COOO• + M 2+ ( M + ) + H + .