Positron annihilation lifetime spectroscopy (PALS) has indicated that Nb2O5-doped TiO2 samples treated in either Ar or air at 1400 degrees C for 48 h are both charge-compensated by Ti vacancies, but with statistically significant differences between the results. A Ta2O5-doped air-sintered TiO2 sample also showed behavior similar to the Nb2O5-doped air-sintered TiO2. In addition, Nb2O5-doped SnO2 samples sintered in air at 1400 degrees C for 1 h showed evidence of Sn vacancy cluster formation, with no indications of Sn2+ compensation from ultraviolet irradiation studies. PALS studies of air-sintered ZrO2 did not show any evidence of Zr vacancies after doping with Nb2O5.