PHOTOCHROMIC materials(1,2) change colour on absorption of light, whereas electrochromic materials(3,4) change colour in response to an electrically induced change in oxidation state. Both classes of materials are being investigated for potential applications in displays, imaging devices and ’smart’ windows(5-8,15,16). Here we describe an alternative route to such applications, in which an electrochromic film and a photovoltaic film form the two electrodes of an electrochemical cell. The resulting structure exhibits photochromism, but unlike conventional photochromic films, the light-absorption process (in the photovoltaic film) is separate from the coloration process (in the electrochromic film) : both may therefore he optimized individually. Moreover, as the coloration process in our cells requires an external electrical current between the two electrodes, the optical state of the cell-transparent, absorbing or, in the case of non-uniform illumination, patterned-can be stored when the circuit is open, or changed when the electrodes are connected.