The formation of beta- and alpha-Fe-3(P2O7)(2) along with Fe-2(PO3OH)P2O7, in iron phosphate-based catalysts for oxidative dehydrogenation of isobutyric acid, has been studied by Mossbauer spectroscopy and X-ray diffraction. It has been shown that the crystallization of beta-Fe-3(P2O7)(2) in the catalysts under the conditions of catalysis is related to the presence in the amorphous precursors of these catalysts of particular ferrous species, whereas the crystallization of alpha-Fe-3(P2O7)(2) occurred when the catalysts were tested under low oxygen partial pressure. From a catalytic point of view, the beta-Fe-3(P2O7)(2) phase appeared to be poorly active and selective and without effect on the catalytic properties of Fe-2(PO3OH)P2O7. On the contrary, the high activity and selectivity of alpha-Fe-3(P2O7)(2) were confirmed. Comparison of the catalytic properties of catalysts containing the same amount of beta-Fe-3(P2O7)(2) along with a large proportion of alpha-Fe-3(P2O7)(2) or Fe-2(PO3OH)P2O7 clearly showed that the catalytic properties of these two last phases were very close. This last feature has been proposed to be correlated with the presence at the surface of both phases under the conditions of catalysis of the same species [(PO3OH)(2-) and (Fe3O12)(16-) Or (Fe2O9)(13-) groups].