Oxidative dehydrogenation of lactic acid to pyruvic acid was performed at 230 degrees C over iron phosphates, with a P/Fe atomic ratio of 1.2, consisting of different structures and oxidation states : these included FePO4, Fe2P2O7, Fe-3(P2O7)(2), and a phase designated as Y-phase. The catalysts consisting of both Fe2+ and Fe3+ ions showed a higher activity than FePO4 and Fe2P2O7. During use in the reaction for more than 24 h, FePO4 was transformed to another unknown phase consisting of both Fe2+ and Fe3+ ions, which is designated here as M-phase. The best performances were obtained with the catalyst consisting of M-phase. The optimum oxygen feed rate to form the M-phase is in the range of 10 to 15 mmol/h, when the feed of lactic acid is 20 mmol/h. The M-phase was stable under the reaction conditions used and was not reoxidized by air even at 300 degrees C.