Phase formation studies in the system Ca2Fe2-xNbxO7 : 0 less than or equal to x less than or equal to 2.0 indicated the presence of two new phases, a cubic solid solution with a variable compositional extent x from 0.45 less than or equal to x less than or equal to 0.65 and an orthorhombic phase for x = 1.0. Cubic solid solution has variable oxygen content with an oxygen-deficient perovskite structure. The oxidation state of Fe was investigated by Mossbauer spectroscopy; as synthesized, for x = 0.6 it contains a mixture of Fe4+ and Fe3+ in the ratio 13/87. Its oxygen content can be modified over the range 5.55 < < 5.89 by heat treatments under a range of conditions. For most values, the crystal structure was that of a cubic perovskite; at high oxygen contents, gamma > 5.7, transformation to an orthorhombic structure was observed. Electrical properties range from semiconducting at high gamma to insulating at low gamma. The orthorhombic phase Ca2FeNbO6 has the GdFeO3 structure. Atomic positions and occupancies were determinated by Rietveld refinements. Mossbauer analysis revealed only the presence of Fe3+ in this sample. Although Ca2FeNbO6 was heated under a range of conditions, its oxygen content could not be modified. The conductivity of Ca2FeNbO6 corresponds to that of an insulator material. Magnetic susceptibility data indicated an antiferromagnetic behavior. (C) 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.