Nickelate-based oxides are potential cathodes in Solid Oxide Fuel Cells operating at intermediate temperatures. The chemical compatibility between apatite electrolyte, i.e., La9Sr1Si6O26.5, and Nd-deficient nickelate. i.e., Nd1.95NiO4+delta has been characterized. The equilibrium between the nickelate material and the gas phase has been studied as functions of temperature and oxygen partial pressure, using potentiometric measurements with microelectrodes, i.e.. metallic or ceramic point electrodes. Two solid electrolytes were used, i.e., apatite and yttria-stabilized zirconia. The response of the nickelate is discussed in terms of oxygen stoichiometry. (C) 2009 Elsevier B.V. All rights reserved.