It was found that sulfur atoms in 4-methyl-1-(methylthiomethyl)-7-thiabicylo[3.3.1]non-3-en-2-one (1) were oxidized in a stepwise manner by potassium iodate in an acidic medium, On the potentiometric titration curve of 1, the first and the second voltage step correspond to the oxidation of the heterocyclic sulfur, which has a higher nucleophilicity and the sulfur in the methylthio substituent, respectively. The equivalent sulfur atoms in 1,1-bis(methylthiomethyl)propan-2-one (2) are oxidized at the same voltage.