This study aimed at characterizing the interactions between a complex peptide mixture and the G-10 membrane (Osmonics, Minnetonka, MN) as influenced by pH. It was previously shown that the composition of the peptide layer interacting with the G-10 membrane and causing fouling was pH-dependent. An adsorption-desorption procedure was used to evaluate the effect of pH (5 and 9) and added CaCl2 (0.01 M) on the profile of peptides adsorbed onto the G-10 membrane. Modifications of the surface properties of the G-10 membrane upon adsorption of tryptic beta-LG peptides were also estimated using contact angle measurements. These methods demonstrated that the nature of the tryptic peptides contained in a beta-LG hydrolysate adsorbing onto the G-10 membrane was influenced by the pH and the presence of Ca2+ ions. The selective influence of the pH was attributed to changes in the ionization state of the membrane. At pH 9, mostly basic (positive) peptides were able to interact with the membrane while at pH 5 no direct correlation with the pI of the peptides was obtained. Hydrophobic interactions were shown to be involved in the adsorption as evidenced with sequence beta-LG 15-20. This peptide is thought to play a key role in the build-up of the fouling layer, determining the resulting sieving properties of the G-10 membrane during NF. (c) 2005 Elsevier B.V. All rights reserved.