Acceptor-doped BaZrO3 exhibits a high bulk proton conductivity combined with good chemical stability. Yet its application as solid electrolyte is severely hampered by its pronounced grain boundary resistance. Results from several experiments described here (including measurements of a sample with 100-200 mu m grain size) strongly support the assignment of this blocking behavior to proton depletion in space charge zones. Approaches for improving grain boundary conductivity based on the space charge model are briefly addressed. (C) 2012 Elsevier B.V. All rights reserved.