This paper reports the effects of heterogeneously doped Al2O3 on the ionic conductivity of yttria stabilized zirconia (YSZ). At lower dopant concentration, grain growth occurred and the grain boundaries were re-formed. Subsequent increases in the dopant concentration decreased the grain size. The doping leads to the creation of space charge regions in the vicinity of the YSZ-Al2O3 boundaries, conducive to enhanced transport of oxygen ions. The presence of Al2O3 also leads to a blocking effect. The net result of the two antagonistic influences is small and reflected by a relatively minor influence on conductivity. (C) 2004 Elsevier B.V. All rights reserved.