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Reduction (in H 2 ) and re-oxidation (in air) behavior of Ca 9 Fe(PO 4 ) 7 was studied by X-ray powder diffraction, Mo ¨ssbauer spectroscopy, thermogravimetry, hydrogen absorption, electrical conductivity, and magnetic-susceptibility measurements. The β-Ca 3 (PO 4 ) 2 -like framework of Ca 9 Fe(PO 4 ) 7 was stable in 100% H 2 up to 820 K. In the temperature range from 680 to 820 K, reversible redox reactions occurred without changing the stoichiometry of oxygen and phosphorus atoms and destroying the structure. The reduction process in Ca 9 Fe(PO 4 ) 7 included the change of the oxidation state of Fe atoms and incorporation of hydrogen atoms into the structure to form Ca 9 FeH x (PO 4 ) 7 (0 < x e 1). Above 820 K, treatment in 100% H 2 was accompanied by loss in mass, partial destruction of the structure, and appearance of FeP and Fe 2 P phases. Re-oxidation studies in air showed that samples partially lost phosphorus atoms during reduction above 820 K. Behavior of Ca 9 Fe(PO 4 ) 7 under a reduction atmosphere depended on H 2 concentration and temperature. At low H 2 concentration, the sample destruction started from ca. 870 K and proceeded with maintaining the oxidation state of Fe, releasing H 2 O, and forming a β-Ca 3 (PO 4 ) 2 -like phase and β-Ca 2 P 2 O 7 . In this case, iron phosphides appeared above ca. 1200 K.