Some thiosulphinates with at least one proton on the carbon adjacent to the sulphenyl sulphur react with aceticanhydride containing acetic acid to afford the corresponding α-acetylthio-sulphoxides. The mechanism of this reaction was studied using thiosulphinates labelled with 2H, 13C, and 18O. These tracer experiments demonstrated that the reaction proceeds via an initial Ei reaction to form the corresponding