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Coloring/bleaching processes of Li x WO3 electrochromism were examined in terms of emf, chronoamperometry, voltammetry and ac impedance measurements. The current-potential relation was found to be ohmic in the short time region and expressed by i=(E app-emf (x))/R. Satisfactory fittings were obtained for chronoamperometry and voltammetry measurements on the basis of the assumption of diffusion-limited process with above i-E relation. Diffusion coefficient was estimated to be about 5×10-10 cm2/sec. It was elucidated from the detailed analysis of impedance that R was given by the summation of the resistances of solution, electrode, charge transfer and material transfer. Since it was found that the WO3 film was regarded as a large condenser with a serially connected resistor, color/bleach processes were understood as charging/discharging processes of WO3 film.