Abstract A thin oxide film has been formed anodically on a tungsten electrode at a potential of 1.0 V (vs. RHE) in 0.1 mol dm −3 sulphuric and perchloric acid solutions. The composition and electrochromic behaviour of the film have been examined by in-situ ellipsometry and Raman spectroscopy. The anodic oxide film has a low refractive index of 1.64 and its in-situ Raman spectrum corresponds to that of a highly hydrated W(VI) oxyhydroxide containing a terminal W O bond. The electrochromic colouring of the film takes place at a potential lower than 0.50 V and the light absorption increases with decreasing potential, where it is directly proportional to the amount of cathodic charge passed during the colouring process. The electrochromic reaction may be initiated by both electron transfer from the metal substrate and proton transfer from the aqueous acidic solution to the oxide. Electrons and protons may be trapped respectively, by tungsten ions and by the terminal oxygens of W=O sites, and the relatively slow change of the Raman signal for the oxide transformation indicates that a relaxation process exists for the formation of W-OH from the W O site and the proton transferred.