TiO2 films were produced by low-temperature reactive magnetron sputtering and characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, electrochemistry, and mechanical-indentation techniques. The TiO2 films consisted of 20-30 nm particles. Electrochemical Li-insertion experiments revealed that the Li diffusion along grain boundaries was much faster compared to the diffusion into the TiO2 grains. The mechanical properties of the films changed drastically after electrochemical Li insertion; the microhardness increase of the Li-inserted TiO2 samples was in the range 70-104% with maximum values ranging between 5.8 and 6.5 GPa. Hardness measurements on a Li-extracted sample showed that the changes in mechanical properties were irreversible. (c) 2007 The Electrochemical Society.