The use of AC impedance spectroscopy for kinetic study of the ion intercalation into WO, films is reviewed, and methods for extracting the diffusion coefficient of the ion diffusion process from AC impedance spectroscopy data are described. These are applied to several different electrochromic thin films, all based on tungsten oxide, and the electrochromic performance is correlated with the diffusion coefficient. The results are also compared with results of a previous paper which concentrated on modelling the voltage response of films coloured and bleached using constant current charge injection techniques. Several examples of non-ideal behaviour of the impedance spectra are observed, including depressed semicircles and evidence of two semicircles. A full discussion of these effects is given in a following paper.