The electrochromic reaction of titanium oxide films prepared by RF sputtering was studied. Chronopotentiometric experiments associated with transmittance measurements in LiClO4/propylene carbonate solutions were carried out for several film thicknesses. The reaction kinetics is controlled by lithium ion diffusion into the titanium oxide matrix. The diffusion equations have been solved for the appropriate finite boundary conditions, and from these analyses l(2)/D values are obtained. Combining these data with coulometric titration data, one can conclude that the diffusion phenomenon is only produced in a part of the film. The I parameter was calculated and a value of 11 nm was obtained, coinciding with the thickness of the accumulation space charge layer for n-type TiO2. Consequently, Li+ diffusion process only occurs in this region.