An in-situ FTIR-ATR method for simultaneously obtaining both kinetic and structural information during liquid sorption into polymers was presented. The kinetics and diffusion profile of the sorption of liquid water and liquid methanol into poly(ethylene terephthalate) (PET) were compared and contrasted. The diffusion of water into PET was shown to follow Fickian kinetics, without significant swelling and the calculated diffusion coefficients (D) varied between 8.57 and 0.52 x 10(-9) cm(2) s(-1) for a crystallinity range of 4-25%. The D values decreased non-linearly with the increase in crystallinity. The average spherulitic crystal size was thought to play a significant role in determining the rate of water sorption. Conversely, the sorption of liquid methanol was accompanied by significant swelling and crystallisation and hence showed non-Fickian or anomalous kinetics. The sorption data were fitted to a dual sorption model which indicated that the rate of diffusion of liquid methanol was faster than that of liquid water, probably due to the accompanying swelling. Increasing the level of crystallinity was shown to decrease the capacity for the polymer to swell and reduced the calculated diffusion coefficients.