In this study, asymmetric cellulose acetate membranes with moderate NaCl rejection (85.5%) were prepared and used to study the influence of the chemical nature of organic solutes in different organic solvents. The solute rejection and the solvent flux of linear hydrocarbons (M-w = 226-563 g/mol) and linear carboxylic acids (M-w = 228-340 g/mol) in ethanol and hexane were studied as a function of the molecular weight, the feed concentration and the transmembrane pressure. The ethanol flux was three times higher than the hexane Aux. The rejection coefficients for both types of solute were quire acceptable (R = 60-90%), when ethanol was the solvent. In hexane the linear hydrocarbons showed a rejection of 40-60%, while all carboxylic acids reached a negative rejection of -40 to -20%. This negative "observed" rejection can be attributed to accumulation of carboxylic acid at the membrane; the solute concentration at the membrane becomes much higher than in the bulk solution, due to a higher affinity of the solute with the membrane in hexane than in ethanol. Sorption experiments support this hypothesis. Furthermore, it was found that the rejection increases with increasing molecular weight and the rejection and Aux are hardly affected by the feed concentration.