Pervaporation (PV) of benzene-cyclohexane and acetone-cyclohexane mixtures through membranes of polyimides containing methyl-substituted phenylenediamine moieties were investigated. The membranes were preferentially permeable to benzene and acetone over cyclohexane due to preferential sorption of benzene and acetone. Polyimides from 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BP) with 2,4,6-trimethyl-1,3-phenylenediamine (3MPD) or 2,3,5,6-tetramethyl-1,4-phenylenediamine displayed high permeation flux, low selectivity and poor membrane durability because of large membrane swelling. Copolyimides from 3MPD and 1,3-phenylenediamine (PD) or 3,5-diaminobenzoic acid (DABA) displayed high PV performance with excellent membrane durability because of the reduction in membrane swelling. For example, BP-3MPD/PD(1/1) copolyimide membranes had a specific permeation flux, Ql, of 2.7 kg mu m/m(2) h and a separation factor, alpha, of 14 at a feed composition of 50 wt% benzene in cyclohexane at 70 degrees C and Ql = 14 kg mu m/m(2) h and alpha = 23 at a feed composition of 68 wt% acetone in cyclohexane at 50 degrees C. The membranes of polyimide from 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BT) and 3MPD were photocrosslinked on UV exposure, resulting in a significant decrease in the permeation flux with a rather,small increase in selectivity. The photocrosslinking was not effective in the PV system