The permeability parameters for moisture transmission through hydrophilic films are well known to be thickness dependent, this effect appearing in permeability studies based on diffusion models. In our model we assume that, besides diffusion, some of the water vapour of the diffusion flux is removed by the hydrophilic components of the films. This water is immobilized and becomes hydration water of the hydrophilic molecules. This paper describes a diffusion-adsorption model, with thickness-independent parameters, which explains this phenomenon.4014146