The water sorption characteristics of poly(ethylene terephthalate) (PET) amorphous samples of 250 mu m thickness have been studied at various temperatures in a saturated atmosphere. Concerning diffusivity, one can distinguish the following two domains characterized by distinct values of the activation energy: E-D approximate to 36 kJ mol(-1) at T > 100 degrees C, and E-D approximate to 42 kJ mol(-1) at T < 60 degrees C, with a relatively wide (60-100 degrees C) intermediary domain linked to the glass transition of the polymer. The crystallization of this latter occurs in the time scale of diffusion above 80 degrees C but doesn't change the Fickian character of sorption curves. The equilibrium concentration m(infinity) is an increasing function of temperature, but the solubility coefficient S decreases sharply with this latter, with the apparent enthalpy of dissolution Delta H-s being of the order of -28 kJ mol(-1) at T < 80 degrees C and -45 kJ mol(-1) at T > 80 degrees C. Density measurements in the wet and dry states suggest that water is almost entirely dissolved in the amorphous matrix at T < 80 degrees C but forms partially a separated phase at T > 80 degrees C. Microvoiding can be attributed to crystallization-induced demixing.