The thermal hysteresis behavior of Matrimid (R) 5218 polyimide membranes in the pervaporation of a tert-butanol and watermixture containing 85 wt.% tert-butanol was investigated. The membrane flux decreases while separation factor increases after the thermal cycle process. The percentage changes of flux and separation factor after cycle are strongly path-dependent. Membrane starting from a high temperature has a more stable performance than that from a low temperature. Three factors play important roles in the hysteresis behavior; namely, interactions between feed molecules and membrane, non-equilibrium nature of dense-selective skin, and asymmetric membrane swelling. As a result, the collapse of micro-porous structure underneath the dense-selective skin and the thickening of the dense-selective skin were observed by SEM pictures after pervaporation. The d-space value measured by WXRD also indicates a reduced interstitial chain space after pervaporation, suggesting a thermal and solvent induced densification. Similar to the asymmetric membranes, a higher flux and separation factor was obtained at 100 degrees C if a dense membrane was tested starting from a low temperature. The interesting behavior may arise from a greater relaxation of polymer chains and an easier cooperative packing of benzene rings induced by the penetrant molecules. As a result, the membrane treated at 100 degrees C has a narrower interstitial chain space and possesses a higher selectivity towards larger molecules transportation. The narrower interstitial chain space and a higher selectivity observed at high temperature may be due to the loss of local chain rigidity and straightness resulting from the nature of Matrimid. (c) 2005 Elsevier B.V. All rights reserved.