Pervaporation (PV) separation of toluene/iso-octane mixtures via copolyimide membranes containing 3,5-diaminobenzoic (DABA) was investigated. Incorporation of the DABA units promotes segmental packing, reduces membrane swelling, and improves membrane selectivity and durability. The material containing 60% DABA-based repeat units provides a normalized flux of 1 kg mum/m(2) h and a selectivity of 90 for the toluene/iso-octane (50/50 wt.%) system at 100 degreesC. The improvement in pervaporation selectivity, relative to the polymer without DABA, is mainly due to the increase in diffusivity selectivity; however, favorable toluene/iso-octane solubility selectivity also contributes significantly as well. A correlation of the pervaporation performance with the intrinsic fractional free volume (FFV) of the glassy polymer is observed and explained in terms of the downstream conditions present in pervaporation. The pervaporation performance of the current polyimides is compared with other materials reported in the literature. Relationships between polymer structure and transport are established. (C) 2003 Elsevier Science B.V. All rights reserved.