A new experimental apparatus for studying the effect of mechanical deformation on the permeation of organic compounds through polymeric membranes is described. The apparatus consists of a custom-built biaxial membrane stretching device and an ASTM F739 permeation cell. Infrared spectroscopy was used as the analytical technique to measure, in real time, the downstream concentration of organic compounds that passed through the membrane. The equipment was used to monitor the permeation of single organic liquids and organic mixtures through uniaxially and biaxially elongated membranes composed of polyisoprene rubber and carbon black. The steady-state flux of acetone through a 40% uniaxially deformed membrane increased by 100%, as compared to that for a nonelongated film, whereas the acetone flux decreased by 25% when the membrane was deformed biaxially 40 x 40%. Variations in membrane thickness with stretching were small and could not account for the observed change in acetone flux with uniaxial elongation. For an equivolume feed mixture of acetone, benzene, and hexane, the steady-state permeation of all three compounds increased when the membrane was elongated 40 x 40%, as compared to the nonstretched case. Some deterioration of the membrane occurred during permeation of pure benzene with biaxial stretching (40 x 40%), as evidenced by a step-jump increase in the benzene flux. No deterioration was observed with nonelongated and uniaxially stretched films.