Equilibrium sorption and uptake kinetics of n-butane and n-pentane in uniform, biaxially oriented, semicrystalline polyethylene terephthalate films were examined at 35 degreesC and for pressures ranging from 0 to approximately 76 cmHg. Sorption isotherms were well described by the dual-mode sorption model. Sorption kinetics were described either by Fickian diffusion or a two-stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at long times. Diffusion coefficients increased with increasing penetrant concentration. n-Butane solubility was lower than that of n-pentane, consistent with the more condensable nature of 12-pentane. However, n-butane diffusion coefficients were higher than those of n-pentane. Infinite-dilution, estimated amorphous phase diffusion and solubility coefficients were well correlated with penetrant critical volume and critical temperature, respectively.