A thermodynamical approach, based on the definition of the chemical potential of water, is used in order to establish a model coupling the diffusion of moisture to the mechanical behaviour of a polymer. The model takes into account the evolution, occurring during the diffusion process, of both the density of the polymer and its maximum moisture absorption capacity. The approach developed is applied to symmetric loading cases, in terms of relative humidity and external pressure. The model, established in the present work, provides the water content profiles through the thickness of the polymer throughout the transient phase of the diffusion process. The numerical results obtained show the significant effect of an external pressure on the kinetics of moisture diffusion in polymers. (C) 2012 Elsevier Masson SAS. All rights reserved.