Investigation of electrical, mechanical a nd microstructural properties of ZnO-NiO two-phase ceramics with respect to the ageing process of the mate ri a I we re performed. Two constituent phases, ZnOss and NiOss were identified in those ZnO-NiO ceramics which exhibit a positive temperature coefficient of electrical resistivity (PTCR anomaly). The origin of the PTCR anomaly was explained as a consequence of the different linear thermal expansion of the ZnOss and NiOss constituent phases. The ageing observed in the material is exhibited as an increase in resistivity of the samples and a smaller PTCR anomaly after heating the samples up to the temperature of the resistivity maximum, and cooling them again to room temperature. According to this study, the different linear thermal expansion of ZnOss and NiOss could also cause ageing of the material.