A series of interval absorption runs in the system poly(methyl methacrylate)-methyl acetate vapor at 30 degrees C exhibiting non-Fickian behavior are presented. The relevant diffusion and viscous relaxation processes were studied separately by kinetic analysis of the first and second stage of two-stage sorption curves, respectively. Both processes were found to be much slower than that observed by Kishimoto et al. (J Phys Chem 1960;64:594) in nominally the same system. The concentration dependence of the diffusion coefficient was found to be well represented by the free volume theory of Vrentas and Duda. The second stage absorption was found to conform well to first-order relaxation kinetics. The corresponding relaxation frequencies exhibit a weak dependence on concentration, in contrast to the strong concentration dependence reported by Kishimoto et al. This difference is attributed to differences in polymer fine structure, reflected in the glass transition temperature of the polymer-penetrant system, which was found to be substantially higher in our case. As a result, Kishimoto et al.'s system undergoes glass transition in the upper region of experimental concentration range, while our system remains glassy throughout. Our data indicate that the S-shaped absorption curves observed at low concentrations are diffusion-, rather than relaxation-controlled.