Tungsten trioxide is the most accepted material for electrochromic devices. In the work thin films of WO3 were deposited by reactive r.f. sputtering of both metallic (W) and ceramic (WO3) targets to study the correlation between the electrochromic properties and the structures of the films. Samples were grown at different pressures of Ar + O-2 in order that the energy regimes of the sputtered particles on the condensing surface could be set either below or above the thermalisation diffusion limit. Lithium ions were intercalated in the films in an aprotic electrolyte and the colouring/bleaching behaviour as a function of the intercalated amount of lithium was detected in the 1st and 10th cycle. From these measurements, the electrochromic properties of the films were worked out. The optical and morphological characteristics were analysed respectively, by spectrophotometric and X-TEM measurements. The amount of water present in the films, detected by IR spectroscopy, turned out to be well correlated to the film morphology and also to the porosity.