Reverse osmosis performance for certain low molecular weight organic solutes in water, such as phenol and its derivatives, is markedly different from that of simple aqueous salt solutions. The unusual solute-membrane affinity reverse osmosis behaviour of phenol-water-cellulose acetate system has been observed. This strong solute-membrane affinity has caused the separation decrease with increasing pressure and permeate flux reduction in the system. This is due to the strong solute-membrane affinity through cellulose acetate (CA) membranes. This work deals with the separation of a mixed organic solute system. A systematic study has been conducted to demonstrate the applicability of reverse osmosis fractionation of diluted aqueous solutions, namely phenol, p-nitrophenol and 3-chlorophenol by asymmetric cellulose acetate membranes. Besides, the effects of operating pressure and feed concentration on the performance of separation are investigated. Results show that the separation can be positive, zero, or negative, depending on the operating pressure, solute concentration as well as the coupling solutes.