The catalytic activity and selectivity of several mixed-valence hydroxyphosphates in the oxidative dehydrogenation of isobutyric acid to methacrylic acid were determined and compared to those of an industrial iron phosphate-based catalyst. The phases were characterized before and after catalytic reaction; after catalysis they appeared to belong to a solid solution of the type Fe4-x3+Fe3x2+(PO4)(3)(OH)(3-3x)O-3x with less than or equal to x less than or equal to 1. The compounds of this solid solution underwent a reversible reduction through a reduction-hydroxylation mechanism without structural change. This mechanism is proposed to be the basis of the catalytic activity of the compounds, which suggests that one of the roles of water added to the gas feed is to displace the hydration equilibrium.