Impregnation of polymers with supercritical CO2 appears to be a promising technique in the area of polymer processing. The main aim of this paper is to measure the diffusion coefficients of a solute (a dyestuff) permeating a poly(ethylene terephthalate) (PET) film in a supercritical impregnating system at different working conditions and to compare the results with similar data obtained with the same solute in the same material but in a traditional aqueous system. The experimental technique used is that of the "film roll method" for the experiments conducted in the supercritical system and has already been tested in a previous work. For the aqueous system, the sorption kinetics of the dyestuff on plane PET sheets has been measured. The results confirm the high gain in the rate of diffusion obtained with the supercritical impregnating system, related to the high plasticizing power of supercritical CO2 toward synthetic polymers. Interesting considerations come also from the comparison of the data obtained by operating with supercritical CO2 at different working pressures and temperatures and in the presence of a proper modifier (ethanol).