Gasochromic windows can change their transmittance over a wide range. This change is caused by a thin layer of tungsten oxide (WO3), covered by a very thin layer of platinum. Exposing this coating to diluted hydrogen gas leads to reduction of the WO3, resulting in colouring. This process can be reversed by introducing diluted oxygen. The hydrogen and oxygen are produced by an electrolyser. Only small amounts of gas are needed for the switching process. The coatings are produced by sputtering. Water is needed in the WO3 films to allow rapid transport of the hydrogen. However, this water should not escape when the system is operated at higher temperatures, which can exceed 60 degreesC. By adjusting the conditions of the sputtering process appropriately, a large amount of water can be incorporated in the films, which remains even up to temperatures above 100 degreesC. The best transmittance values obtained for a coated double-glazed unit with a moderate film thickness (560 nm) and hydrogen concentrations below the combustion limit are 76% and 77% for solar and visual transmittance, respectively, in the bleached state and 5% and 6% for solar and visual transmittance, respectively, in the coloured state. Darker states can be obtained by applying thicker films of tungsten oxide without reducing the transmittance in the bleached state. Gasochromic coatings can also be deposited easily on plastic substrates and-because their coating structure is so simple-combined with prismatic micro structures, which allow light to be redirected. In addition, the gasochromic systems technology can be used with metal hydride systems. (C) 2004 Elsevier B.V. All rights reserved.