The perovskite type oxides of the system Sr-Ce-Fe-O were prepared by mixing and thermal treatment in air. Microstructure and phase composition were determined by scanning electron microscopy and X-ray diffraction. Besides the pure SrCeO, and SrFeO,_, a solid solution range of Sr,_YCe,FeO,_Z was observed between y = 0.01~ 0.2. This solid solution is the active phase responsible for high oxygen exchange and electrical conductivity as required for cathode materials of high-temperature electrochemical cells. The oxygen exchange and oxygen deficiency n were measured by solid electrolyte coulometry. From pO,-T-x diagrams the partial molar enthalpies and entropies of oxygen were calculated for the air-oxidized states of the solid solution and of the two-phase region: AH, of the two-phase material 0.8 SrCeO,-0.2 SrFeO,_, is -23 to -29 kJ mol-' and AS, is -3 to -8 J mol-' K-'. For the solid solution Sr0.9Ce0,1Fe0,_, the values depend on x: AH, increases from -33.5 to -28.4 kJ mol -' and A&, increases from -10 to + 20 J mol-' K-' in the range 0.24 < x < 0.36. The dc electrical conductivity of the monophase material Sr, _YCe,FeO,_, show a maximum near 500°C at yc. = 0.05 mol (80 S cm-', E, = 0.088 eV). The conductivity is of p-type in the region of higher oxygen partial pressures and n-type at reducing conditions.