Heterogeneous catalysis by MnO2 of oxalic acid ozonation in aqueous solution has been investigated in the pH range 3.2-7.0. The influence of catalyst load and reaction temperature upon rate development of the ozonation process has also been examined. The observed dependence of system reactivity on pH and the results obtained by ozonation experiments with MnO2 catalysts of different pH(zpc) values agree with a reaction mechanism involving the formation of a surface Mn-oxalic acid complex.