Sputtered WO3 films were coated with platinum and coloured by exposure to diluted hydrogen gas. Films with a high water content show a fast exponential increase of the optical density with time, whereas films with a low water content show a square root dependence, which is typical for slow diffusion. The combination of several experiments led us to a mechanism for the diffusion: the WO3 layer consists of columns with surfaces, which are covered by water. The hydrogen gas is dissociated by the Pt, and reacts with the water to form H3O+ and electrons, which reduce the WO3. The protons diffuse via the formation of H3O+ along the water on the interior WO3 surface.