An in-depth study of sorption, swelling, and permeation of aqueous ethanol, 2-propanol (IPA), and tert-butyl alcohol through BTDA-TDI/MDI (P84) co-polyimide membranes has been conducted. The superior separation performance toward aqueous IPA and tert-butyl alcohol mixtures mainly arises from four factors: (1) rigid P84 chains with a small d-space, (2) the lesser affinity between P84 and IPA as well as between P84 and tert-butyl alcohol, (3) a high sorption selectivity, and (4) steric hindrance induced by bulky IPA and tert-butyl alcohol molecules. However, because of severe chain swelling and relaxation in ethanol/water solutions as confirmed by XRD and DSC, P84 performs less impressively for the pervaporation dehydration of ethanol/water mixtures.