Mossbauer spectroscopy has been used to characterize several hydroxyphosphate catalysts used in the oxidative dehydrogenation of isobutyric acid into methacrylic acid. It was found that under the conditions of catalysis all the tested phases were transformed into phases belonging to a solid solution of the type Fe4-x3+Fe3x2+(PO4)(3)(OH)(3-3x)O-3x with 0 less than or equal to x less than or equal to 1. Depending upon their composition, these phases contained clusters of face sharing iron octahedra of different sizes of which the largest ones underwent a fast electron transport process. This process which gave rise to the observation by Mossbauer spectroscopy of mixed-valent cations, affected the catalytic properties of the solids by both increasing their selectivities for acetone and carbon dioxide at the expense of that for methacrylic acid and decreasing their total activity. From these results together with previously published data, it is suggested that the presence of the mixed-valent cations lowers the basicity of the O2- ion involved in the reaction. The change in activity seemed to be related to the change in selectivity between parallel reactions whose rates depend upon the mobility of H+ or O2- within the solid.