Mixed conducting oxides of Nd 1.8 Ce 0.2 Cu 1−x Ni x O 4+δ for varying values of x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9 were synthesized by combusting mixed precursor materials under microwave heating. The Nd 1.8 Ce 0.2 Cu 1−x Ni x O 4+δ solid solutions are tetragonal structures in one single phase across the composition range, but it is convenient to divide the range into tetragonal Nd 2 CuO 4 — cuprate for compositions corresponding to x≤0.5 and while for x>0.5, the same phase is described as tetragonal Nd 2 NiO 4 — nickelate compositions. In this Nd–Ce–Cu–Ni–O system, the maximum d.c. conductivity σ=0.619Scm −1 , at a selected temperature of 700°C, was obtained at a composition corresponding to x=0.5, represented by the chemical formula Nd 1.8 Ce 0.2 Cu 1−x Ni x O 4+δ . At this composition, the lowest value of activation energy (E a =0.11eV) was measured over a temperature range of 500 to 700°C. Symmetric cells in the configuration given by— electrode/electrolyte/electrode—, with Nd 1.8 Ce 0.2 Cu 1−x Ni x O 4+δ (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 0.44Ωcm 2 was obtained at a temperature of 700°C, for the solid solution corresponding to x=0.5 and directly correlating with the maximum in d.c. conductivity.