Impedance spectra were measured for two types of lanthanum-substituted PbWO4 systems (Pb1-xLa2x/3WO4 and Ph1-xLax-WO4+x/2), from which the relaxation phenomena associated with oxide ion conduction were investigated. Profiles of the imaginary part of the electric modulus normalized at the peak position were not essentially varied with temperature or composition for Pb1-xLa2x/3WO4 system, whereas broadening of the modulus peak was observed for Pb1-xLaxWO4+x/2 system. We attributed this broadening to the localization of oxide ion interstitials since it appeared only for the latter system and the peak broadening grew larger with decreasing temperature. (c) 2006 Elsevier Ltd. All rights reserved.