Membrane compaction accounts for a major source of the flux decline that occurs during gas separation using polymeric membranes. Previous membrane compaction studies have been limited by the inability to simultaneously measure membrane performance. In this study, we report the development of a technique based on ultrasonic time-domain reflectometry (TDR) that enables the simultaneous, real-time, noninvasive measurement of membrane compaction and performance during gas separation. In order to demonstrate the utility of the technique, representative data are presented for membrane compaction and pressure-normalized flux of a commercial asymmetric cellulose acetate (CA) gas separation membrane. Data obtained during the recovery cycle, i.e., after the pressure difference across the membrane is removed, are also described. These preliminary results indicate that the ultrasonic TDR technique can be successfully applied to quantify the effect of high-pressure gases on polymeric films and asymmetric membranes.