A commercial thin film composite polyamide reverse osmosis membrane is used to separate an aqueous phenol-water binary system. The separation data are analyzed using a combined film theory-solution-diffusion (CFSD) model and a combined film theory-Spiegler-Kedem (CFSK) model. In the present investigation a new phenomenon is observed: there exists a maximum in the rejection when it is plotted against the product flux through the membrane. This behavior is explained for both models. An equation for J(v,min), which is the value of the product flux J(v) at which the rejection reaches a maximum, is derived from both models. Although the parameters for both models are consistent over the range of operating conditions, the CFSK model is more accurate for the phenol-water system.