ReO3-type solid solutions in the Nb2O5-WO3 system were synthesized by heat treatment of niobium-doped peroxo-polytungustic acids at relatively low temperature ranging from 600 to 1000 degrees C. The resultants cooled from 1000 degrees C to room temperature were monoclinic in the composition range 0 less than or equal to x less than or equal to 0.05 [x = Nb/(Nb + W)] and tetragonal for 0.05 < x < 0.25; there was a mixture of two tetragonal phases in the range x > 0.25. The Rietveld structural refinement showed that a tetragonal compound at x = 0.118 takes a distorted ReO3-type structure [a = 5.2961(5), c = 3.8534(4) Angstrom at 25 degrees C], analogous to the high-temperature form of WO3, in which some oxygens are vacant and the metallic elements are distributed at random. Chemical insertion of lithium into such a tetragonal host framework was carried out using a 0.016 M n-butyl lithium/n-hexane solution at 25 degrees C. It accommodated Li up to y = 1.0 in terms of LiyNbxW1-xO3-x/2 (x = 0.118), but the host structure remained tetragonal in the whole range of y.