The sorption and kinetics of acetone uptake in solvent cast films of poly(ethylene terephthalate) are reported at 35 degrees C for acetone pressures ranging from 0 to 7.3 cm Hg. The equilibrium sorption isotherm is well described by the dual-mode sorption model with the following parameters: k(D) = 61 cm(3)(STP)/(cm(3).atm), C-H＇ = 7.2 cm(3)(STP)/cm(3), and b = 50 atm(-1). Sorption kinetics are described using a two-stage model which incorporates both Fickian diffusion and protracted polymer structural relaxation. The characteristic time associated with the relaxation process is essentially independent of acetone concentration and has an average value of approximately 15 hours. The fraction of sorption associated with polymer relaxation increases linearly with acetone concentration in the equilibrium-densified matrix of the polymer. Acetone diffusion coefficients increase with increasing acetone concentration. The concentration dependence of the acetone diffusion coefficient is well described by the dual-mobility model if the assumption of constant diffusion coefficients in the two modes is relaxed.