A kinetic study of the heterogeneous catalytic hydrogenation of model polystyrenes (PS) using a catalyst comprised of Pt on a wide-pore silica support was performed. The effect of molecular weight on the initial rate of hydrogenation, r0, was investigated under conditions where mass transfer limitations were minimized. The r0 values for a series of model polystyrenes ranging in molecular weight from 1.5 to 276 kg/mol were measured by monitoring hydrogen uptake with a digital mass flow controller. The initial rate of hydrogenation was found to be inversely proportional to PS molecular weight. For molecular weights up to 102 kg/mol, r0 scaled with the number-average degree of polymerization, X̄n, to the -0.15 power. The two highest molecular weight samples, 190 and 276 kg/mol, exhibited significantly slower initial rates of hydrogenation and did not follow this trend. These results are explained in terms of equilibrium structure and conformational dynamics of a polymer on a metal surface and the relative dimensions of the polymer coils and Pt crystallites.