The kinetics of Os(VIII) catalysed oxidation of L-leucine by diperiodatoargentate(III) (DPA) in alkaline medium at 298 K and a constant ionic strength of 0.60 mol dm(-3) was studied spectrophotometrically. The oxidation products in both the cases are pentanoic acid and Ag(I). The stoichiometry is, i.e., [L-leucine]:[DPA] = 1:2. The reaction is of first order in Os(VIII) and [DPA] and has less than unit order in both FL-leu] and [alkali]. The oxidation reaction in alkaline medium has been shown to proceed via a OS(VIII)-L-leucine complex, which further reacts with one molecule of MPA in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test and spectral studies. The reaction constants involved in the different steps of the mechanism are calculated. The catalytic constant (K-c) was also calculated for Os(VIII) catalysis at different temperatures. From the plots of log Kc versus I/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. The active species of catalyst and oxidant have been identified. (C) 2007 Elsevier B.V. All rights reserved.