This paper discusses the results of the analysis of products distribution and selectivity in hydrogen peroxide (H2O2) epoxidation of maleic acid (MA) in the presence of sodium tungstate as catalyst in the native form and also immobilized on polymer resin support. A specially designed experimental procedure afforded accurate monitoring of the oxygen released during epoxidation occasioned by H2O2 decomposition and also enabled selectivity to be reliably estimated. Irrespective of the catalytic system employed, it was observed that between 10 and 15% of the H2O2 initially present in the reaction is lost via decomposition which, presumably, is induced by the action of the metallic catalytic species. Consequently, selectivity was observed to drop gradually in the course of the reaction, but more rapidly in the case of the homogeneous catalysis.