The separation of gas mixtures using polysulfone hollow-fiber membrane cells was studied experimentally. Three gas mixtures (O-2/N-2, CO2/N-2/O-2, CH4/CO2/N-2) were used in the study. The variables examined include the flow configuration, feed pressure, and reject flow rate. The results indicate that the extent of separation in terms of the separation factor as defined by McCandless [7] is strongly in influenced by the feed pressure and reject flow rate. Furthermore, the separation factor was found to be independent of the stage cut for the cocurrent case, but was found to increase rapidly if the flow was countercurrent. (C) 1997 Elsevier Science B.V.