In this book a unique protocol for selecting, evaluating, characterizing, and modifying potential Solvent Resistant Polymeric Nanofiltration (SRNF) membranes for organic solvent separation as well as elucidating some of the underlying principles are presented. Organic solvent separation systems are encountered in industries including the petroleum, petrochemicals, pharmaceutical, biotechnology and fine chemicals where polymeric membranes and if not selected or modified properly may result in very low performances as results of possible excessive swelling and compaction. Ultrasonic Time Domain Reflectometry (UTDR) is involved for in-situ and real-time chemical (swelling) and mechanical (compaction) analysis as well as characterization while the membrane is permeated with an organic solvent or feed of interest. Performance parameters used to assess the membrane include flux, rejection, separation factor, solute and solvent relative recoveries and relative solute loss. These performance parameters provide an approach to access and monitor the effects of operating conditions on both solute purification and solvent recovery for re-use.