The catalytic effect of molybdovanadophosphate heteropolyanions (HPAs) on the delignification of wood by oxygen has been investigated. The VO(?)2 ions produced via the dissociation of HPA in acidic conditions are suggested to have a much stronger catalytic effect on the oxidation of lignin than parent HPA. The influence on delignification of the pH, medium composition and the partial reduction of HPA prior to its use in catalysis are discussed in terms of the reaction mechanisms. Kinetic studies show that the catalyst diffusion in the wood tissue is the rate-limiting step of delignification, while the lignin oxidation rate is determined by the HPA/VO2+ catalytic oxidation step.