We examine the space charge layer formation at metal/ionic conductor interfaces by solving Poisson's equation using defect energetics calculated from first principles. The scheme is applied to zirconia, and we find that oxidizing atmosphere and high valence band offset result in negative space charge accumulation at the interface, while reducing atmosphere and low valence band offset result in positive charge accumulation. This is explained in terms of the alignment of the Fermi levels in the metal and the ionic conductor. (C) 2010 Elsevier B.V. All rights reserved.