We present an analytical solution for non-steady-state diffusion in coated materials which assumes a constant gradient in the coating. The application domain and degree of validity are determined as a function of two appropriate parameters. Possible surface and interface effects are taken into account, as far as is consistent with boundary conditions of the first kind. The solution applies generally to thin films in which the diffusion equation prevails, whether to describe diffusion of a species or heat transfer by conduction. For example, this solution provides an easy tool for studying diffusion processes in multilayer materials in situ which is well adapted to the case of protective coatings.