Oxygen gas was observed by means of a mass spectrometer to be evolved from calcium-doped ceramic cerium oxide when direct current was passed through it at 450°C. The applied voltage necessary to cause the electrolysis was about 100 V across a sample 4 mm long containing 0.86 at. % calcium. When voltages of 1.0 V or less were applied to ceramic and single-crystal samples there were pronounced transients and nonlinear potential distributions. These latter effects appear to be consistent with a picture of oxygen ions migrating to an electrode where they are at least partially blocked.