The effect of boundary faceting on densification has been studied in a 5 mol.% TiO2 excess BaTiO3 model system. It was possible to inhibit grain growth during sintering in moderately reducing atmospheres after a hydrogen presintering treatment and, thus, to exclude the effect of grain growth on densification. For a given oxygen partial pressure (P-O2) there was a saturated relative density which decreased with increasing P-O2 and boundary faceting. The fraction of small pores was also higher in the samples sintered under high P-O2 than in those sintered under low P-O2. These experimental observations demonstrate the presence of a critical driving force for densification in systems with faceted boundaries and its dependence on boundary faceting, i.e. the critical driving force is larger for systems with greater faceting. The result further confirms that a faceted boundary, either fully or partially faceted, is an imperfect atom source for densification; this idea is in contrast to the conventional understanding. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.