The kinetics of oxidation of ruthenium(III) (Ru(III)) and osmium(VIII) (Os(VIII)) catalysed oxidation of L-phenylalanine (L-Pal) by diperiodatoargentate(III) (DPA) in aqueous alkaline medium at 27 A degrees C and a constant ionic strength of 0.25 mol dm(-3) was studied spectrophotometrically. The involvement of free radicals was observed in the reactions. The reaction between DPA and L-Pal in alkaline medium exhibits stoichiometry as [L-Pal]:[DPA] = 1:1. The reaction is of first order in [Os(VIII)], [Ru(III)] and [DPA] and has negative fractional order in [IO4 (-)]. It has less than unit order in [L-Pal] and [OH-]. However, the order in [L-Pal] and [OH-] changes from first order to zero order as their concentrations increase. The main oxidation products were identified by spot test and spectral studies. The probable mechanisms were proposed and discussed. The catalytic constant (K (c)) was also calculated for Os(VIII) and Ru(III) catalysis at different temperatures. The activation parameters with respect to slow step of the mechanisms were computed and discussed and thermodynamic quantities were also calculated. It has been observed that the catalytic efficiency for the present reaction is in the order of Os(VIII) > Ru(III). The active species of catalyst and oxidant have been identified.