The features of a new type of smart window are evaluated. The ＇gasochromic＇ device uses a WO, layer that can be coloured by performing a reaction with gaseous hydrogen via a catalyst. The kinetics of this reaction as a function of several parameters were determined fbr optimization purposes: The dependence of the colouration speed on the H-2 concentration was found to be linear for the range 0.1-20%. The temperature dependence is of the Arrhenius type with a Gibbs energy of activation of 27 kJ/mol. For evaporated layers, the kinetics depend strongly on the overall thickness of the catalyst layer. The shape of the curve for colouration versus time is different for layers with different microstructures i.e. diffusion coefficients. A model accounting for these effects was developed. Within this model the kinetics are described by a surface-controlled process (such as the adsorption) and one-dimensional diffusion of protons and electrons within the bulk material. We found that diffusion limits the speed for dense materials, while the surface process is the limiting one for porous materials. SEM (secondary electron microscopy) images of the examined layers reveal that not only the degree of amorphicity influences the diffusion, but also the structure on a scale of 100 nm (columns formed in the deposition process). The influence of the water content of the atmosphere and the layer on the kinetics and degradation of the device are also discussed. The samples are produced by e-beam evaporation, reactive magnetron sputtering and reactive gas flow sputtering (RGFS) causing the different microstructure of the WO3 layer.