The glass transition temperature of a series of ethylene-vinyl alcohol copolymers which have been exposed to methanol is determined by dynamic mechanical measurements as a function of methanol concentration. The results of a number of sorption and desorption measurements taken in the temperature range 21-60 degrees C for the same polymer-solvent system are also reported. An interpretation of these data, which relates these two sets of results, is presented. This interpretation differs considerably from those currently available in the literature for sorption data similar to those reported here. The need to deconvolute from the sorption data the effect of macroscopic elastic constraints arising during the swelling process and in particular to distinguish features of the sorption curves which reflect true material properties of the system as opposed to simple geometrical effects is pointed out. Once this is done, the main qualitative features of our results, namely, initial sorption similar to root t, presence of sharp concentration fronts during sorption, and existence of two different desorption regimes, can be accounted for on the basis of a simple description of solvent transport based on Fick’s law with a diffusion coefficient which changes suddenly from a low value characteristic of the glassy state to a high value typical of rubbery polymers at the concentration at which plasticization takes place. The geometry dependent features of the sorption curves can also be understood within this framework.