A kinetic model is developed for the ozonation of oxalic acid catalyzed by solid MnO2. The rate of ozonation is limited by the adsorption of oxalic acid on tl-re catalyst surface and hy the deactivation of a fraction of the active sites, because of an iri eversible reaction with ozone, Moreover, the model includes the ozonation of oxalic acid catalyzed by dissolved manganese. This kinetic model allows for a fairly good correlation of the experimental data when the average size of the catalyst particles is smaller than about 10 mu m. When larger particles are employed, noticeable mass-transfer limitations are encountered, mainly deriving from the diffusion of both reactants from the liquid bulk to the solid surface (external and internal), The dependence of the rate constant on pH is experimentally determined and explained in terms of the changing chemical structure of the active sites and of the dissociation equilibrium of oxalic acid.