The transfer of oxygen from transition metal oxidants to sulfur (MO4− + R2S → MO3− + R2SO) has been studied using the oxidation of alkylthio or arylthioacetic acids (RSCH2CO2H) by perruthenate as models. These reactions exhibit a first-order dependence on the concentrations of both the oxidant and the reductant, and are directly proportional to the basicity of the medium. The Hammet ρ value is negative (ρ = −0.66), a better correlation being obtained when σ, rather than σ+, substituent constants are used. The entropies of activation for the reactions are large and negative, indicating that the transition state is highly structured. A reaction mechanism involving an activating expansion of the ruthenium coordination shell through incorporation of a hydroxide ion is proposed. Oxygen transfer is then initiated by reaction of a nonbonding pair of sulfur electrons with either a vacant rutheniumd-orbital or a Ru=O π*-orbital.Keywords: perruthenate, sulfides, oxygen transfer, oxidation.