Photoelectrochromic windows represent a special kind of switching windows. The energy for colouring is provided by sunlight, so that a voltage supply is not required. The transmittance can be decreased on illumination and can be increased again in the dark. In contrast to photochromic devices, the system is externally switchable under illumination. Our photoelectrochromic window consists of several components: a dye-covered nanoporous TiO2 layer, which is situated on a nanoporous electrochromic layer, such as WO3, two glass substrates coated with a transparent conductive oxide, of which one is coated with Pt, an iodide/tri-iodide redox couple and Li+ ions in a solid ion conductor. All the layers can be kept quite thin, so that they are transparent. The pores of the TiO2 and WO3 layers are filled with the electrolyte. This configuration is a particularly advantageous combination of the dye solar cell and an electrochromic element. The colouring time is independent of the area, the transmittance can be varied also in the illuminated state, and the system can also be switched by an auxiliary external voltage. Initial samples with solid electrolyte change their visible transmittance from 62% to 1.6%, their solar transmittance from 41% to 0.8%. The time for colouring and bleaching is about 15 min. (c) 2005 Elsevier B.V. All rights reserved.