The construction of an electrochromic window based on a modified transparent nanostructured metal oxide film (TiO2 anatase, 4.0 mu m thick) supported on conducting glass (F-doped tin oxide, 10 Ohm/square, 0.5 mu m thick) is described. The nanostructured TiO2 film is modified by adsorption of a monolayer of the redox chromophore bis-(2-phosphonoethyl)-4,4＇-bipyridinium dichloride, the electrolyte is 0.05 M LiClO4 and 0.05 M ferrocene in gamma-butyrolactone and the counter electrode is conducting glass. The performance of the above device (colouration efficiency of 170 cm(2) C-1 at 608 nm, switching time of 1 s and stability over 10 000 standard test cycles) suggests an electrochromic technology with commercial potential.