Asymmetric membranes based on P84 co-polyimide were prepared via a wet phase inversion technique. The thermodynamic and kinetic properties of the ternary nonsolvent/NMP/P84 systems were studied and correlated with the membrane structures. The effects of the membrane preparation conditions, viz., nonsolvent type, polymer solution concentration, casting solution thickness, evaporation time and coagulation temperature on the resulting membrane structures were analyzed. The morphology of the cross-sections of the membranes were observed using scanning electron microscopy (SEM) and the skin surface morphology of the membranes were studied using tapping mode atomic force microscopy (AFM). The porosity and BET surface area of the membranes were also measured. The gas (He, CO2, O-2 and N-2) permeation tests on the resulting asymmetric membranes were carried out and the thickness of the skin layer was also determined. The asymmetric membranes with several hundred nanometres of dense selective skin layers were successfully prepared. It was found that the selectivities of gas pairs (He/N-2, CO2/N-2 and O-2/N-2) were higher than those of dense P84 membranes and increased with decreasing the skin thickness of the asymmetric membranes. The asymmetric membrane prepared from the acetone/NMP/P84 chemical mixture exhibited the best gas separation performance with permeance of 17, 1.8, 0.82 and 0.092 GPU for He, CO2, O-2 and N-2, respectively, and with selectivities of 200, 20 and 9 for He/N-2, CO2/N-2 and O-2/N-2, respectively. (C) 2012 Elsevier B.V. All rights reserved.