The objective of this study was to investigate the physico-chemical parameters that determine the retention of phenolic compounds in aqueous solution by nanofiltration membranes. To this purpose. fundamental properties of two membranes (NF90 and NF270) were determined from their observed performance. Molecular weight cut-offs (MWCO) were determined using crossflow filtration of different molecular weights of polyethylene glycol (PEG). Their mean pore size was also estimated by the steric hindrance pore (SHP) model using neutral organic solutes (saccharides). The salt retention sequence of membranes was determined using single aqueous solutions of Na2SO4, NaCl and CaCl2. Retention of phenolic compounds by NF270 was lower than that of NF90, because of its larger pore size. The influence of the solute adsorption upon the NF90 membrane retention was described by the molecular hydrophobicity of organic compounds. Finally, the effect of solution chemistry upon the membrane retention of phenol and resorcinol was investigated at different pH values. Near the isolelectric point of the membranes, minimum organic retention and maximum flux were observed due to membrane electroneutrality. (C) 2008 Elsevier B.V. All rights reserved.