Infrared Fourier Transform phase-modulated and UV-visible ellipsometry techniques have been applied to study the colouring/bleaching process of WO3 electrochromic thin films. The optical absorption of these films can be changed markedly in a reversible process induced by hydrogen incorporation. The colouring/bleaching process was carried out by using the sample as one electrode of an electrochemical cell filled with an electrolytic solution of H2SO4. The spectroscopic ellipsometric angles psi and Delta were obtained ex situ by an infrared Fourier Transform phase-modulated ellipsometer working in the wavenumber range from 800 to 4000 cm(-1) and a rotating analyzer ellipsometer in the UV-visible range (1.7-4.8 eV). Significant optical absorption and vibrational changes associated with the colouring/bleaching process which follow the hydrogen incorporation in the film were observed in the UV-visible range from 1.7 to 3.8 eV and in the IR range at 1605 cm(-1) and from 3000 to 3800 cm(-1). The strong changes can he seen by changes in the spectroscopic dielectric function. For the coloured WO3 layer, the main absorption contribution associated with the colour effect was found at 6032 cm(-1), outside the analysed region.