Owing to the positively charged grain boundary cores in acceptor-doped ZrO2 and CeO2, oxygen vacancies are depleted in the space charge layers. The validity of this space charge concept was checked for Y2O3-doped CeO2 ceramic of high purity. Electrical fields up to 2 x 10(5) V cm(-1) were applied to the grain boundaries of 1.0 mol % Y2O3-doped CeO2 at 400 degrees C in air, and the grain boundary properties were separated by means of impedance spectroscopy. It was discovered that the current-voltage relation for individual grain boundary was nonlinear, and that the effective grain boundary thickness increased with increasing bias, which supports the space charge concept. (C) 2004 The Electrochemical Society.