A series of monodisperse, sterically stabilized colloids of electrocatalytically active ruthenium dioxide hydrate (RuO2.XH(2)O) with diameters between 40 and 160 nm are used to examine the relationship between particle size, catalyst concentration, and the heterogeneously catalyzed rate of water oxidation by cerium(IV) ions in dilute nitric acid. Reaction rate is linearly dependent upon catalyst concentration for all particle sizes. Rate constants vary with particle radius raised to the power -1.7 and exhibit absolute values consistent with mass transfer of Ce-IV ions to the catalyst surface being the rate-limiting step. Activation energies are found to decrease with particle radius, and this effect is attributed to the influence of the steric stabilizer upon the temperature-dependent viscosity of solution near the catalyst surface.