A novel process mode for gas separation by means of membranes was presented. It involved cyclic pressure-vacuum swings for feed pressurization and permeate evacuation, thereby increasing the transmembrane pressure difference and the feed to permeate pressure ratio. The feed pressurization and permeate evacuation were preferably done using a single pump capable of pressurization and suction. Unsteady-state permeation associated with the pressure-vacuum swing permeation was analyzed to elucidate the working principle of the process, and a parametric study was carried out to evaluate the effects of design and operating parameters on the separation performance. To exemplify the feasibility and effectiveness of the process, air separation for oxygen enrichment was investigated as a model application. It was shown that the separation efficiency was enhanced by means of the pressure-vacuum swing, and both product purity and product throughput higher than those from conventional steady-state permeation could be obtained. (C) 2014 Elsevier B.V. All rights reserved.