The kinetics of ruthenium(III)-catalyzed oxidation of sulfamethoxazole by heptavalent manganese in alkaline medium it a constant ionic strength of 0.40 mol dm(3) has been studied spectrophotometrically using a rapid kinetic accessory. The reaction between permanganate and sulfamethoxazole in alkaline medium in presence of ruthenium(III) exhibits 2:1 stoichiometry (KMnO4:sulfamethoxazole). The reaction shows first order dependence on [permanganate] and [ruthenium (III)] and apparent less than unit order dependence each in sulfamethoxazole and in alkali concentrations. Reaction rate decreases with increase in ionic strength and decrease in solvent polarity of the medium. Initial addition of reaction products did not affect the rate significantly. A mechanism involving the formation of a complex between catalyst and substrate has been proposed. The activation parameters were computed with respect to the slow step of the mechanism. The reaction constants involved in the mechanism were evaluated.