Structure refinements have been carried out on pure and lanthanide-substituted PbWO4 using powder neutron diffraction, and the defect structures of La- and Pr-substituted PbWO4 are discussed. The Rietveld refinement structure of pure PbWO4 agreed well with previous results. In the Pb1-chiLachiO4+chi2 system, the oxide ion interstitials seemed to be rather localized around z=0.3 of the 8e position (0 0 z) or the adjacent general position of the scheelite structure-type with I4(1/a) symmetry. The deduced interstitial position is considered in terms of the oxide ion conduction path. A similar defect structure has been determined for the praseodymium-substituted compound, Pb1-chiPrchiWO4-chi.(2). In contrast, Pb1-chiLa2chi.3WO4-type substitution resulted in the formation of a cation-deficient structure. (C) 2002 Elsevier Science B.V. All rights reserved.