The electrical properties of 9 mol% MgO-ZrO2 (Mg-PSZ) with 1 mol% Al2O3 and the mechanisms for electrical degradation were investigated using structural, morphological, and electrochemical analyses. The addition of Al2O3 caused an increase in both the monoclinic and the Mg-rich phases at the grain boundaries in the Mg-PSZ. Coarse grains larger than 20 mu m and an intergranular layer composed of the Mg-rich phase were identified in a specimen sintered at 1600 degrees C. This specimen exhibited a minimum of ionic conductivity (4.98 x 10(-4) S cm(-1) at 700 degrees C) due to the grain boundary resistance (245 Omega cm(2)), which dominated the overall resistance. A similar trend was observed over the entire temperature range (600-1500 degrees C). An intergranular siliceous impurity (SiO2) was present in conjunction with the Mg-rich phase. This impurity and the Mg-rich phase acted as a barrier layer for oxygen ion diffusion. The presence of the intergranular phases (i.e. the monoclinic and Mg-rich phases) contributed to the degradation of the ionic conductivity in Mg-PSZ with an Al2O3 addition. 2014 Elsevier B.V. All rights reserved.