It was discovered experimentally that heteropolymolybdophosphoric acids (HPA) with Keggin and Dawson structure are inactive for H2O2-decomposition, while their salts (Fe3+, Cu2+, Co2+ and Mn2+) all possess more activity. It could be concluded that the active species is the countercation of these kinds of heteropolymolybdates. The molybdenum ions in the polyanions are not active. If the molybdenum ions in the polyanions of these acids are substituted partly by vanadium ion (HPA-n), then not only does the catalytic activity increase regularly with the number of vanadium ions substituted (n) but the kinetic curve is also different from that of the salt, characterized by an S-shape, indicating the formation of an active intermediate as a result of the reaction between the polyanion of HPA-n and the substrate.