A major limitation in previous compaction studies has been the inability to obtain direct simultaneous measurements of permeate flux and membrane thickness changes in real-time. In this paper we describe the development and application of ultrasonic time-domain reflectometry (TDR) for quantifying membrane compressive strain. The non-invasive nature of the technique allows standard performance data including permeate flux to be simultaneously measured under realistic conditions. Representative data for commercial reverse osmosis membranes are presented to demonstrate the potential benefits of this more complete approach to compaction studies. Results are also described for ultrasonic TDR experiments using cellulose acetate membranes which show the effect on membrane compressive strain behavior of changes in overall porosity and upstream pressure. Additional results show the variation in compressive strain during both the creep and recovery phases of a single pressurization cycle as well as over multiple cycles. Given these capabilities, the use of ultrasonic TDR should enable improved experimental and modeling analyses regarding the effects of operational and structural parameters on membrane performance.