The grain boundary cores of acceptor-doped zirconia and ceria are positively charged. which is probably due to the oxygen, vacancy enrichment there. Oxygen vacancies are therefore depleted in the space charge layers. which is the decisive cause of the low grain boundary conductivity in acceptor-doped zirconia and ceria materials of high purity. This space charge concept is elaborated. and crucial experimental evidences are summarized. (C) 2004 Elsevier B.V. All rights reserved.