Aiming the optimisation of nanofiltration (NF) for the concentration of ultrafiltration (UF) permeates from industrial fermentation broths, concerning clavulanic acid yield and permeation flux, the present work focused on the study of the electrokinetic properties of membranes in order to get insight of the mechanisms underlying the NF process. Streaming potential determinations showed that the membranes tested (NFT50, Desal DK and Desal DL) were negatively charged at pH 4.5-5.0, which is the actual pH of NF feed. Nevertheless, the isoelectric point (IEP) varied significantly with the electrolyte type, ionic strength and membrane pre-treatment revealing the importance of these variables upon the membranes electrokinetic properties. The increase of membranes surface charge density for increasing concentration of salts (KC], K2SO4, potassium clavulanate and MgSO4) was correlated by Freundlich adsorption isotherms. The role of sieving exclusion in the NF process was demonstrated for potassium clavulanate whereas electrostatic exclusion was the dominant mechanism for KCl. Adsorption of negatively charged species contained in UF permeates from fermentation broths increases the membranes surface charge density, although their JEP remain constant. (c) 2006 Elsevier B.V. All rights reserved.