Up to 5 mol% Al2O3 was added to 9 mol% Y2O3-stabilized ZrO2, and the roles of Al2O3 were systematically studied by means of the complex impedance approach, the positron annihilation technique, SEM, TEM, and electron probe microanalysis from the following aspects : (1) the existence of forms of Al2O3 in ZrO2, (2) the effects of Al2O3 on the microstructure of ZrO2, (3) the effects of Al2O3 on the resistance of ZrO2, (4) the microstructure and property changes of ZrO2 with Al2O3 addition during ageing at 940 degrees C. Two types of grain boundary phase, crystal and amorphous, were discovered. The Al2O3 segregation at grain boundaries can promote the mobility of the grain boundaries and thus results in a low density, because of entrapped pores. The Al2O3 addition decreases the grain-boundary resistance in two ways : to scavenge the SiO2 and CaO located at grain boundaries, and to form crystal grain-boundary phases with very high crystal defect concentrations. Ordered microdomains of Zr3Y4O12 were precipitated from ZrO2 grains during ageing, and aluminium was found to facilitate the precipitation.