Yttria-stabilized zirconia (YSZ) thick films and bulk ceria (CeO2) ceramics used as membrane electrodes are shown to work as high temperature pH sensors. The YSZ film electrode shows a nearly ideal Nernstian response of the potential as a function of pH with mV/pH = - 56.8 even at 353 K. The YSZ film electrode responded to pH changes within 30 s, while the response time of the samaria-doped ceria ceramic membrane electrode was 3 min. Complex impedance analysis revealed that the bulk conductivity of the ceria ceramics is higher than that of YSZ film, whereas the grain-boundary conductivity of the former was 10(2) times lower than that of the latter. Based on these results, it appears that at temperatures below 600 K, the pH sensory characteristics of these materials are dominated by the grain-boundary conductivity.