In general, grain boundaries in acceptor-doped BaZrO3 are blocking for excess protons, oxygen vacancies and holes. One possible explanation for this blocking grain boundary character is the presence of an excess positive charge in the grain boundary core causing depletion of positive charge carriers in the adjacent space charge zone. A pertinent test consists in the investigation of the grain boundary effect on excess electrons. The vanishing of the grain boundary resistances in strongly reduced, n-conducting Y-doped BaZrO3 ceramics observed in the present study is thus a strong evidence for the space charge picture. Based on this, strategies for diminishing the grain boundary resistivity for positive carriers are outlined briefly. (C) 2011 Elsevier B.V. All rights reserved.