Nonstoichiometric WO3-λ thin films, with oxygen deficiencies, λ, varying between 0.05 and 0.01, were prepared by RF sputtering. These values were determined by a method of chemical analysis consisting in the oxidation of the tungsten ion with less charge than +6 toW6+ by means of a KMnO4 solution. The electrochromic process inWO3-λ / MxWO3-λ, whereM= H, Li, Na and 0.01 ≤ x ≤ 0.08, occurs reversibly. It is believed that the rate-controlling step of coloration is the diffusion of cation M+ into WO3-λ thin films. The cation diffusivity in the thin films was measured as a function of their λ and x values by means of a current-pulse- relaxation technique. The diffusivities, D, were about 5 × 10 -10 cm2 s-1, 2 × 10-10 cm 2 s-1, and 1 × 10-10 cm2 s-1 for H+, Li+, and Na+, respectively, in the stoichiometric films at 300 K. D values increased with an increase of oxygen deficiency and slightly increased or decreased with an increase of cation content. The pulse technique used may probably be responsible for the higher diffusivities obtained, as compared with results for similar systems provided by previous workers.