In this study, the electrical conductivity of undoped cerium oxide is evaluated using impedance spectroscopy. We have observed a new phenomenon, which is not related to electrode response but probably to diffusion processes in the grain boundary. An equivalent circuit has been constructed based on the observation, and the electronic bulk, ionic grain, and ionic grain boundary conductivities are calculated. Evaluation of fitting results to the proposed equivalent circuit suggests that diffusion-related phenomena are caused by blocking effects in the grain boundaries due to impurity segregation. Investigation of the grain boundary composition by energy dispersive X-ray spectroscopy shows that it contains detectable amounts of impurities in comparison to the grain, which supports the suitability of the proposed equivalent circuit. (c) 2005 The Electrochemical Society. All rights reserved.