Equilibrium sorption and uptake kinetics of i-butane, i-pentane, methyl acetate, and ethyl acetate vapor in uniform, biaxially oriented, semicrystalline poly(ethylene terephthalate) films were determined at 35 degreesC and low penetrant activity (or relative pressure). Sorption isotherms of i-butane, methyl acetate, and ethyl acetate were well described by the dual-mode sorption model. The sorption isotherm of i-pentane was described by the dual-mode model at low activity and the Flory-Huggins model at high activity. At low penetrant activity, the solubility coefficients of i-pentane and i-butane were, respectively, 11.5 and 1.3 times lower than those previously reported for their linear analogues. At constant penetrant activity, the solubility coefficients of the acetates were substantially higher than those of the alkanes, and the solubility of methyl acetate was higher than that of ethyl acetate. The sorption kinetics were described either by Fickian diffusion or by a two-stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at, long times. The diffusion coefficient for each penetrant increased with increasing penetrant concentration. The diffusion coefficients of the esters (methyl acetate and ethyl acetate) were quite similar to those of the ketones (acetone and methyl ethyl ketone) with the same number of carbon atoms.