The cubic perovskite phase Ca₂Fe_2–xNb_xO_γ(0.45 <x< 0.65) has been prepared by solid-state reaction at 1400 °C followed by rapid cooling to room temperature. On slower cooling, or heating below 1360 °C, it decomposes to a mixture of Ca₂Fe₂O₅ and Ca₂FeNbO₆, but the quenched single-phase material is kinetically stable up to at least 950 °C. The oxygen content of composition x= 0.60, determined by thermograviinetry, can be varied over the range 5.55<γ<5.89 by heat treatments under a range of conditions, including H₂–Ar at 700 °C to obtain low γ, and high-pressure oxygen, 420 bar at 950 °C, for high γ. The main oxidation state of Fe, as determined by Mössbauer spectroscopy was Fe³⁺, but about 13 % Fe⁴⁺ was present in the as-prepared sample, increasing to 42% under high-pressure O₂. For most γ values, the crystal structure was that of a cubic perovskite with Fe/Nb disorder on the B sites and some oxygen vacancies; at high γ contents, an orthorhombic distortion gave a structure resembling that of Ca₂FeNbO₆. Electrical properties range from semiconducting at high γ consistent with Fe³⁺/Fe⁴⁺ exchange, to insulating at low γ; although there was no direct evidence of oxide ion conduction over the range 25–300 °C, the rapid adjustment of oxygen content γ on changing the atmosphere at 700–900 °C indicated significant mobility of oxide ions. The phase is therefore likely to be a mixed oxide ion–electron conductor.