Abstract: The catalytic behaviour of 12-molybdophospboric acid and its salts in the decomposi-tion reaction of H₂O₂ has been studied by comparing them with the conventional Fenton reagent.The results showed that 12-molybdophosphoric acid gives only negligible catalytic activity, while its salts have the activity comparable to that of the Fenton reagent. It is suggested that the active species were metallic counterions from heteropolyacid salt. The activity of 12-molybdophosphoric acid was markedly affected by substitution of Mo⁺⁶ in the polyanion by V⁺⁵ and increased regularly with the number of V⁺⁵ substituted. For the decomposition reaction of H₂O₂ by 12-molybdophosphoric acid a S-shaped kinetic curve was given rather than a parabolic one as in the case of Fenton reagent. The S-shaped curve is characteristic of branch chain reaction and indicates the formation of more active intermediate as a result of the reaction betwcen V⁺⁵ in the active center of polyanion and the substracte.