The non-Fickian sorption kinetics of methanol vapor in poly(methyl methacrylate) films 8 and 51 mu m thick at 25 degrees C are presented. The behavior of the system was studied in series of interval and integral absorption runs. The relevant diffusion coefficient and viscous relaxation processes were studied separately by kinetic analysis of the first and second stages of sorption kinetic curves. The sorption isotherm concaved upward at high activities, this being typical of Flory-Huggins behavior, whereas it exhibited a convex-upward curvature at low methanol vapor activities, this indicating sorption in the excess free volume of the polymer matrix. After excess free-volume fill-up, the concentration dependence of the diffusion coefficient was found to be well represented by the free-volume theory of Vrentas and Duda. Relaxation frequencies calculated from the second stage of two-stage curves exhibited a weak dependence on the concentration. Integral sorption experiments indicated that the system exhibited nearly case II kinetics at high methanol vapor activities. (c) 2005 Wiley Periodicals, Inc.