In order to improve the performance of a nanofiltration membrane (DS5 DL from Osmonics (R)) several pre-treatments have been tested. Specifically, the membrane has been immersed, for different times, in an aqueous solution of HF or H3PO4, or of a mixture of both these acids. The chemical and charge properties of the membranes, both pre-treated and untreated, have been studied. The chemical changes have been analyzed by ATR-IR and SEM-EDX. These changes can be interpreted in terms of a partial hydrolysis. Moreover, the charge of the nanofiltration membranes has been analyzed in terms of the Donnan steric partitioning pore model with dielectric exclusion, DSPM-DE. Results on the charges inside the pores (volume charge) are compared with those coming from streaming potential from tangential measurements and referred to the membrane surface, done as a function of salt concentration and pH. These results on charges allow an evaluation of the adsorption parameters and the proper charge that are interpreted in terms of the chemical modifications induced by a partial hydrolysis. These changes in chemical nature and charge enable proper interpretation of the performances of the membranes in the elimination of metallic impurities in industrial phosphoric acid solutions. The acidic pre-treatment can also be applied to improve the performance of the membrane in other applications. (c) 2007 Elsevier B.V. All rights reserved.