Changes in the structural and surface properties of 12-molybdophosphoric(abbreviated as PMo₁₂) and 10-molybdo-2-vanadophosphoric(abbreviated as PMo₁₀V₂) acids as they are decomposed at high temperatures have been studied by IR, XRD, XPS, and TPD experiments. PMo₁₂ simply decomposes into MoO₃ by heat-treatment at 580℃, but PMo₁₀V₂ yields a mixed oxide whose properties are different from those of either MoO₃ or V₂O₅. After the thermal decomposition, PMo₁₂ loses the initial acidic and redox properties almost completely but PMo₁₀V₂ retains some extent of the redox capability that is larger than that of MoO₃ and V₂O₅. The redox capability of the mixed oxide obtained by PMo₁₀V₂ decomposition is correlated with its catalytic activity in oxidation processes.