P84 polyimide membranes with thicknesses ranging from 6 to 310 lm were successfully fabricated by spin coating. The glass transition temperature of the P84 powder was found to be 315 C using differential scanning calorimetry, whereas its decomposition temperature was 536 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 CO 2 ) and the ideal selectivity of gas pair (O 2 /N 2 , He/ CO 2 , CO 2 /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/CO 2 , CO 2 /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 mor-phology and gas transport properties were investigated for three annealing temperatures, i.e., 80 C, 200 C, and 315 C. The membrane annealed at 315 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 lm thickness) annealed at 80 C were 16.2, 0.196, 1.20, and 2.01 Barrer for He, N 2 , O 2 , and CO 2 , respectively. The permeabilities of P84-118 membrane annealed at 200 C decreased by 9.75%, 47.96%, 25.83%, and 30.85% for He, N 2 , O 2 , and CO 2 , respectively, as compared with those at 80 C, whereas the ideal selectivities increased by 42.65%, 30.52%, 32.85%, and 21.63% for O 2 /N 2 , He/CO 2 , CO 2 /N 2 , and He/O 2 , respectively. V