We report on the photoluminescence properties of Sc2O3- or In2O3-doped yttria-fully-stabilized zirconia (YSZ) by sub-bandgap photo-excitation to localized electronic states of oxygen vacancies, in order to clarify the mechanism for the aging-induced decrease of ionic conductivity via vacancies known in YSZ and suppressed by such doping. A new band emerged at 2.70 eV and remained even after the aging for the Sc2O3-doped samples at the doping concentration whereof the conductivity decrease was suppressed. On the other hand, a sharp and singular increase of the photoluminescence, intensity was observed for the In2O3-doped samples at the range of the dopant concentration wherein a singular conductivity drop was observed and the suppression of conductivity decrease started. It is suggested that the suppression mechanism is different between Sc2O3-doped YSZ (Sc-YSZ) and In2O3-doped YSZ (In-YSZ). (C) 2007 Elsevier Ltd. All rights reserved.