Mixed conducting oxides of Nd1.8Ce0.2Cu1 - xNixO4 + delta far varying values of x = 0.0, 0.1, 03, 0.5, 0.7 and 0.9 were synthesized by combusting mixed precursor materials under microwave heating. The Nd1.8Ce0.2Cu1 - xNixO4 + delta solid solutions are tetragonal structures in one single phase across the composition range, but it is convenient to divide the range into tetragonal Nd2CuO4 - cuprate for compositions corresponding to x <= 0.5 and while for x > 0.5, the same phase is described as tetragonal Nd2NiO4 - nickelate compositions. In this Nd-Ce-Cu-Ni-O system, the maximum d.c. conductivity sigma = 0.619 S cm(-1), at a selected temperature of 700 degrees C, was obtained at a composition corresponding to x = 0.5, represented by the chemical formula Nd1.8Ce0.2Cu1 - xNixO4 + delta. At this composition, the lowest value of activation energy (E-a = 0.11 eV) was measured over a temperature range of 500 to 700 degrees C. Symmetric cells in the configuration given by- electrode/electrolyte/electrode-, with Nd1.8Ce0.2Cu1 - xNixO4 + delta (for x = 0.3, 0.5 and 0.7) as the electrodes were fabricated by spin coating. The minimum area specific resistance (ASR) value of 0.44 Omega cm(2) was obtained at a temperature of 700 degrees C, for the solid solution corresponding to x = 0.5 and directly correlating with the maximum in d.c. conductivity. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.