Asymmetric hollow fiber membranes provide economical and environmental friendly separations compared to conventional separations (e.g. distillation). They have been widely used in various applications such as nitrogen enrichment and natural gas purification. One of the current challenges of polymeric membrane technology is to provide separations in aggressive environments such as supercritical conditions. Under these conditions, the membrane should not only withstand high pressures but also be robust against swelling agents. To meet these requirements, both the choice of the membrane material and the morphology of the membrane play a crucial rule. We have chosen Torlon®, a polyamide-imide polymer, as the membrane material because its robustness against various organic solvents and its high mechanical strength make it suitable for aggressive environments. Defect free skin and macrovoid free substructure are preferred morphological aspects of the membrane for high pressure applications. The above preferred morphology is obtained successfully by choosing appropriate dope and bore fluid compositions based on the thermodynamic (phase transition) and kinetic aspects (precipitation rates) of fiber spinning. Effects of these morphological features on the permeation and selectivity of the membrane will be illustrated.