In this work, we developed and validated a non-destructive and expedient ellipsometry method for the direct characterization of interfacially-polymerized (IP) selective layers in thin-film composite membranes, such as those used in reverse osmosis (RO) or nanofiltration (NF). The primary advantages of this method are direct analysis of membrane samples and avoidance of IP layer isolation. IP layer isolation has proved to be extremely useful in the past but is also laborious, requires hazardous solvents, and carries significant risks of damage or alteration of the layer morphology if not done correctly. The new ellipsometry method was used to characterize IP layer morphology, including thickness and roughness, and produced results that agree fairly well with measurements by scanning/transmission electron microscopy and atomic force microscopy. Moreover, given the non-intrusiveness of the ellipsometric measurements, dynamic in-situ studies of RO membranes exposed to fluids were demonstrated. Findings from this study are expected to catalyze the further development and application of ellipsometry as a tool for characterizing TFC membrane selective layers and observing their real-time responses to process fluids, chemical and physical stimuli (pH, temperature, etc.), and cleaning agents.