Non-Fickian sorption kinetics of methanol vapor in a poly(methyl methacrylate) film of 8 mu m, at 35 degrees C, are presented. The behavior of the system was studied in series of interval absorption runs. The relevant diffusion and viscous relaxation processes were studied by kinetic analysis of the sorption kinetic curves, using the relaxation-dependent solubility model. The sorption isotherm concaves upward at high activities, typical to Florry-Huggins behavior, while it exhibits a convex-upward curvature at low methanol vapor activities, indicating sorption in the excess free volume of the polymer matrix. Thermodynamic diffusivity presents a complex functional dependence on the concentration, while relaxation rate is found to be a function of concentration as well as of concentration interval. Relaxation rate becomes increasingly concentration-dependent as the effective glass transition of the system is approached. (c) 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3173-3184, 2006.