The effect of NiO addition into gadolinium doped ceria (GDC) on the phase stability, reduction behavior and electrical properties were investigated. X-ray diffraction studies showed that ceria and gadolinia formed a solid solution whereas NiO forms a composite phase with GDC. X-ray diffraction patterns of all these composites were indexed and their cell parameters were generated. The electrical properties of the ceria gadolinia solid solution were determined in the temperature range 400-560 degrees C and those of NiO/GDC composites between 90 and 135 degrees C by impedance spectroscopy. Temperature programmed reduction (TPR) was performed to investigate the reducibility of the NiO/GDC composite. TPR profiles can be attributed to four different states of NiO species due to different interactions between NiO and GDC. (c) 2005 Elsevier B.V. All rights reserved.