The life of Nafion-coated WO3 electrochromic films in 0.5 mol dm (H2SO4)-H--3 was studied by cyclic voltammetry and in situ UV spectrophotometry and compared with uncoated films. The presence of Nafion film at the electrode surface restricted the diffusion of tungsten ions to the bulk electrolyte, thereby significantly prolonging the life of the electrodes. This technique should find useful applications to other catalytic electrodes in aqueous media.