A model for spiral wound reverse osmosis modules FT30-4040 has been developed by analyzing stimulus response experiments. Perfect mixing cells represent edthe flow and dispersed plug, flow represented the membrane. The, model was based on phenomenological equations describing the permeation. The identification of the Peclet number and the space-time characterizing each dispersed plug flow led to the estimation of the tracer dispersion coefficient in the membrane. Ones new and two used composite reverse osmosis modules Were tested. The decrease of the rejection proper-ties of the used membranes-was linked to the apparition of new plug flows with high dispersion coefficients within the membrane.