The sorption/desorption of carbon disulfide into/from PEEK as a function of crystallinity and temperature was investigated. The sorption curves of carbon disulfide into PEEK show only two major regions : (a) an increase of penetrant weight with time, and (b) a limiting equilibrium value (solubility). This is in contrast to the sorption of toluene into PEEK which shows three regions. The solubility of carbon disulfide decreases with increasing crystallinity, but temperature has little effect on the solubility in the temperature range of 25-40 degrees C. An acceleration in penetration rate at the later stage of diffusion is observed for PEEK films whose crystallinity is greater than 13.4%, suggesting Supercase II diffusion. Carbon disulfide can be desorbed completely from PEEK in contrast to other fluids, such as toluene or methylene chloride, which are difficult to desorb. The normalized weight loss of carbon disulfide during desorption is an exponential function of square-root time. Solvent-induced crystallization was observed. Crystallinity was estimated from both the measured density and microhardness of the desorbed polymer and polymer which had undergone a sorption/desorption/resorption/desorption cycle.