Oxidation kinetics of L-citrulline (Cit) by hexachloroplatinate(IV) (HCP) was studied spectrophotometrically in perchloric acid solutions in the presence of ruthenium(III) catalyst at a constant ionic strength of 1.8 mol dm-3 and at 20°C. The reaction rate was very slow in the absence of the catalyst. The reaction showed first order kinetics in both [HCP] and [Ru(III)] and less than unit order with respect to both [Cit] and [H+ ]. Increasing ionic strength and dielectric constant were found to increase the oxidation rate. Both spectral and kinetic evidences revealed formation of an intermediate complex between Lcitrulline and ruthenium(III) prior to the rate-determining step. The complex reacts with the oxidant (HCP) by an inner-sphere mechanism leading to decomposition of the complex in the rate-determining step to give rise to the final oxidation products of L-citrulline which were identified by both spectroscopic and chemical tools as 4-(carbamoylamino) butyraldehyde, ammonia and carbon dioxide. The rate-law expression for the catalyzed reaction was deduced. The reaction constants involved in the different steps of the reaction mechanism have been evaluated. The activation parameters of the second order rate constant have been evaluated and discussed