The relaxation-dependent solubility model was applied to simulate the experimentally observed sorption kinetic behavior of the PMMA-vapor MeOH system at 25 degrees C, to obtain the main transport parameters. Application of the model in series of successive sorption kinetic runs covering small concentration intervals revealed certain trends in the concentration dependence of the diffusion coefficient of the system, not detectable by a previous Simpler analysis of the data. Following excess free volume fill up, relaxation frequencies exhibit a weak exponential dependence on concentration. The functional dependence of the thermodynamic diffusivity on the concentration, deduced from the aforementioned simulation procedure, was tested and found to reproduce reasonably well the series of sorption kinetic runs covering considerably larger concentration intervals. In addition, this analysis indicates a strong dependence of the relaxation mechanism on the concentration interval. (c) 2006 Wiley Periodicals, Inc.