Water-soluble colloidal manganese dioxide has been used to oxidize L-tyrosine in aqueous-acidic medium. The kinetics of the reaction was studied in the absence and presence of non-ionic surfactant (TX-100) using a spectrophotometric technique. As the reaction was fast under pseudo-first-order conditions ([L-tyrosine]>>[MnO(2)]), the rate constants as a function of [L-tyrosine], [MnO(2)], [HClO(4)] and temperature were obtained under second-order conditions. The rate of the reaction increased and decreased with the increase in [L-tyrosine] and [MnO(2)], respectively. Perchloric acid, sodium pyrophosphate and sodium fluoride showed catalytic effect. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. The reaction is inhibited by the non-ionic surfactant TX-100. Activation parameters have been evaluated using Arrhenius and Eyring equations. Based on observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO(2) followed by one-step two-electron transfer rate limiting process.