Films of WO3 were coated with platinum and coloured by exposure to diluted hydrogen gas. Bleaching was achieved with diluted oxygen. Because of its simple layer structure, this so-called gasochromic device is suitable for lame area window applications. By variation of several parameters, such as film thickness and gas concentration, and by considering the bleaching in vacuum and argon, we found a model for the mechanism of the reaction. It is strikingly different to the ideas of the double injection model, and there are indications that the gasochromic and electrochromic reactions are similar for porous films. According to our model, H-2 is dissociated on the platinum, transferred into a pore or grain boundary of the WO3 and creates water and an oxygen vacancy. The O vacancy diffuses into the interior of the grain and the water slowly leaves the film. The bleaching in argon or vacuum is assumed to be the reverse reaction. The bleaching by oxygen is suggested to occur by by the dissociation of the O-2 on the platinum, transfer into a pore and recombination with the oxygen vacancy. (C) 2000 Elsevier Science B.V. All rights reserved.