Gas flows in laboratory membrane test cells are often assumed to be uniform due to the relatively small feed volumes and ideal mixing of gas mixtures such as air. Scientific literature on characterization of gas flow distribution in the feed volume above the membrane in laboratory cells is limited. Any non-uniform distribution of flow is known to impact the permeance through the membrane [V.V. Ranade, A. Kumar, Comparison of flow structures in spacer-filled flat and annular channels, Desalination 191 (2006) 236; Nikhil Kawachale, Ashwani Kumar, Deepak M. Kirpalani, Numerical Investigation of Hydrocarbon Enrichment of Process Gas Mixtures by Permeation through Polymeric Membranes, Chem. Eng. Technol. 31,(1) (2008) 58]. In this work, the conventional cell used in laboratories was first examined to determine the uniformity in flow distribution and its effects on membrane performance. Using a combination of experimental and numerical methods, the feed flow distribution in the conventional cell was found to be skewed towards the far (retentate) side of the feed volume and also resulted in short-circuiting of the gas mixture to the retentate outlet. A new design for laboratory cell was proposed and experimentally validated. This modified cell was found to have a more uniform flow distribution than the conventional cell, consequently leading to improved separation performance. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.