The polarographic reduction of phenacyl phenyl su@hoxide gives rise to two well-defined waves in an ethanol buFTer solution of pH 7 in the presence of Triton X-100, the half-wave potential of the first wave is less negative by about 0.28 V than that of phenacyl phenyl sulphide, the second one coincides with that of acetophenone, the wave-height ratio of the first to the second is closely 2, and the height of the first wave corresponds to a four-electron reduction. Controlled potential coulometry shows that the n-values are 3.5 and 5.6 at potenials of the first and second waves, respeclively. The macroelectrolysis at the first wave potential gives phenyl disulphide, benzenethiol, and acetophenone, while at the second wave potential the reduction products of acetophenone are additionally obtained. These indicate that the electrolytic reduction of the /I-keto sulphoxide proceeds uia neither the corresponding S-hydroxy sulphoxide nor 8-keto sulphide, but causes the cleavage of the S-C bond of the sulphinylmethyl group (-S&CH,-) at the initial step.