The microstructure and electrical properties of YSZ (ionic)-NiO (electronic) composites were investigated. The electrical properties of the composites were examined using impedance spectroscopy between 160 and 630 degrees C. The effects of grains and grain boundaries on the electrical conductivity of the composites vary with composition and temperature. With increasing temperature, the contribution of grain boundary to the total resistivity decreases for YSZ-rich compositions (NiO less than or equal to 30 mol%). For NiO-rich compositions (NiO > 50 mol%), the contribution of grain boundary resistance is variable with temperature, showing maxima near 530 degrees C. Above 630 degrees C, the grain boundary resistance is small for all compositions. With varying composition, grain boundary resistance is greater when the number of YSZ/NiO contacts is greater than the number of YSZ/YSZ or NiO/NiO contacts below 500 degrees C. The temperature- and composition-dependence of grain and grain boundary conductivity is explained by the microstructure and thus by the distribution of the two phases together with variations in activation energy according to the composition.