Defect structures of the lanthanum-substituted PbWO4 prepared by mechanical alloying (MA) were investigated by means of powder neutron diffraction and FT-IR, to compare with that of the oxide ion conductive sintered samples. Neutron diffraction revealed that Ph1-xLaxWO4 + (x/2) prepared by MA possessed the lead site deficiency in the lattice. In addition, it was also found that occupation factors of the lead site were still smaller than the nominal value even for the mechanically alloyed Pb1-xLa2x/3WO4. The vacancy formation at the lead site of the mechanically alloyed Pb1-xLaxWO4 + x/2 was also corroborated from the splitting of the absorption band around 800 cm(-1) ascribed to the W-O stretching vibration Of WO4 tetrahedron. At the end of this paper, the formation process of the lead site vacancy was discussed in the sintered Pb1 - xLaWO4 + x/2 during milling. (C) 2004 Elsevier B.V. All rights reserved.