Lithium insertion to distorted ReO3-type metastable solid solution NbxW1-xO3-x/2 (0 less than or equal to x < 0.25) has been studied by chemical and electrochemical methods. In the course of lithium insertion into tetragonal compounds, transition to a cubic phase was found to occur in the region where values of y tin LiyNbxW1-xO3-x/2) fall between 0.2 and 0.3, and the phase transition was found to depend on the conditions of the reaction. Changes in OCV and lattice parameters in tetragonal region (y < 0.2) were discussed from the viewpoint of the ordering of lithium ions. Also, the component diffusion coefficient of lithium in tetragonal compounds Li0.1NbxW1-xO3-x/2 (0 less than or equal to x less than or equal to 0.23) was found to increase with niobium content when x less than or equal to 0.10, and to saturate at 4 X 10(-9) cm(2)/s.