P84 polyimide membranes with thicknesses ranging from 6 to 310 pm were successfully fabricated by spin coating. The glass transition temperature of the P84 powder was found to be 315 degrees C using differential scanning calorimetry, whereas its decomposition temperature was 536 degrees C using thermogravimetric analysis. Scanning electron microscopy was used to examine the morphology of the membranes. The permeability of single gas (He, N-2, O-2, and CO2) and the ideal selectivity of gas pair (O-2/N-2, He/CO2, CO2/N-2, and He/O-2), as a function of membrane thickness, were determined. The results showed that the permeability of a single gas increased with increasing membrane thickness, whereas the selectivity of a given gas pair was nearly independent of the membrane thickness. The average selectivity of O-2/N-2, He/CO2, CO2/N-2, and He/O-2 were found to be 8.2, 10.0, 12.9, and 15.8, respectively. The effects of heat treatment on the membrane morphology and gas transport properties were investigated for three annealing temperatures, i.e., 80 degrees C, 200 degrees C, and 315 degrees C. The membrane annealed at 315 degrees C was cracked due to the stress sustained either during heating or cooling, thereby resulting in little or no selectivity. The permeabilities of P84-118 membrane (118 pm thickness) annealed at 80 degrees C were 16.2, 0.196, 1.20, and 2.01 Barrer for He, N-2, O-2, and CO2, respectively. The permeabilities of P84-118 membrane annealed at 200 degrees C decreased by 9.75%, 47.96%, 25.83%, and 30.85% for He, N-2, O-2, and CO2, respectively, as compared with those at 80 degrees C, whereas the ideal selectivities increased by 42.65%, 30.52%, 32.85%, and 21.63% for O-2/N-2, He/CO2, CO2/N-2, and He/O-2, respectively. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2906-2912, 2010