Absorption and desorption of organic vapors in amorphous polymers in the temperature region slightly above Tg were studied as a function of film thickness. The systems chosen for this study were polyvinyl acetate (PVAc)–allyl chloride at 40°C. and polymethyl acrylate (PMA)–methyl acetate at 15 and 35°C. In PVAc studies reduced adsorption curves for different film thicknesses at relatively high equilibrium concentrations were apparently normal, but the initial slopes of the plots increased with increasing film thickness X and appeared to approach certain limiting values at the limit of infinite X. At lower equilibrium concentration two-stage absorption curves appeared, and the variation of absorption curve with film thickness was too complicated to be interpreted in terms of the theory proposed by Crank and Long and Richman. The desorption curve was at all concentrations studied, almost normal and the initial slopes increased with increasing X. By assuming that the hypothetical sorption curve with the initial slope extrapolated to infinite X is controlled by a purely Fickian diffusion mechanism, we calculated the mutual diffusion coefficient D for this system and compared it with that derived by Meares from steady-state permeation measurements. Both results agreed quite well, suggesting that the method presented here is effective for obtaining D. Absorption and desorption of methyl acetate by PMA at 15°C. were entirely similar to those obtained by PVAc in the corresponding region of equilibrium concentration. At 35°C. reduced sorption curves were independent of X and were purely Fickian. Following the analysis in PVAc, we predicted D for PMA at 15°C. as a function of methyl acetate concentration.