The kinetics of Os(VIII) and Ru(III) catalysed oxidation of anti-pyretic drug, aspirin by diperiodatoargentate(III) (DPA) in alkaline medium at 298 K and a constant ionic strength of 0.10 mol dm(-3) was studied spectrophotometrically. The oxidation products in both the cases are 1,4-benzoquinone2-carboxyl ate ion and Ag(I). The stoichiometry is the same in both the catalysed reactions i.e., [aspirin]:[DPA] = 1:2. The reaction is of first order in Os(VIII)/Ru(III) and [DPA] and has less than unit order in both [ASP] and [alkali]. The oxidation reaction in alkaline medium has been shown to proceed via a Os(VIII)/Ru(III)-aspirin complex, which further reacts with one molecule of DPA in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test, IR, NMR and GC-MS. The reaction constants involved in the different steps of the mechanism are calculated. The catalytic constant (K.) was also calculated for both catalysed reactions at different temperatures. From the plots of log K-c versus 1/T, values of activation parameters with respect to the catalyst have been evaluated. The activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also determined. It has been observed that the catalytic efficiency for the present reaction is in the order of Os(VIII) > Ru(III). The probable active species of catalyst and oxidant have been identified. (c) 2006 Elsevier B.V. All rights reserved.