In gravimetric experiments, the swelling and the drying of polymer films is used to investigate the thermodynamic properties and the mutual diffusion coefficient of polymer/solvent systems. Usually thermodynamic equilibrium at the interface between the film and the solvent vapor and thermal equilibrium between the film and the surroundings are assumed. In this paper we show that the second assumption may fail. Indeed, during a swelling or drying experiment, the temperature of the film surface changes due to the latent heat of vaporization, which induces a variation of the activity. When the corresponding variation of the solvent content is of the same order than the variation due to the sorption experiment and when the thermal time constant is significant compared to the characteristic mass diffusion time, this thermal effect must be taken into account when analyzing sorption data. We evaluate the consequence of this thermal effect on gravimetric experiments and develop a complete model to take this phenomenon into account when analyzing sorption data. As an example, the mutual diffusion coefficient for the system PIB (polyisobutylene)/toluene is estimated for various solvent concentrations at 25 ° C. © 2005 Elsevier Ltd. All rights reserved.