Iron phosphates are effective as catalysts for oxidative dehydrogenation of lactic acid to pyruvic acid and that of glycolic acid to glyoxylic acid unlike phosphates of vanadium and molybdenum. The structure of iron(III) orthophosphate is transformed into a new phase having crystalline water during the use in these reactions. This induces a marked increase in the catalytic performances. The formation, characteristics, and catalytic properties of the new crystalline phase were studied.