Abstract The freshly calcined iron phosphate catalysts with a P/Fe atomic ratio of 1.0 ∼ 1.3 consist of amorphous phase, quartz-type FePO 4 and/or tridymite-type FePO 4 depending on the calcination temperature, though they contain only Fe 3+ ions. In the absence of oxygen, the P/Fe > 1.2 catalyst samples are reduced with hydrogen or isobutyric acid to form Fe 2 P 2 O 7 via Fe 3 (P 2 O 7 ) 2 . While the P/Fe = 1.0 samples are reduced to Fe 2 P 2 O 7 not passing through Fe 3 (P 2 O 7 ) 2 . Fe 2 P 2 O 7 is oxidized with air to an unknown compound (Y phase), but not to Fe 3 (P 2 O 7 ) 2 nor FePO 4 . The Y phase is reduced to Fe 2 P 2 O 7 . During the use in the oxidative dehydrogenation of isobutyric acid, the freshly calcined catalysts are reduced to Fe 3 (P 2 O 7 ) 2 , the Fe 2 P 2 O 7 catalysts are in part oxidized to the Y phase, and the Y phase is in part reduced to Fe 2 P 2 O 7 , though the rates are very slow. The catalytic activity and selectivity are scarcely affected with the variation in the structure of the iron phosphate catalyst and in the oxidation states of iron ions.