The aim of this study is to investigate to what extend the addition of an electrolyte (NaCl or Na(2)SO(4)) can improve the selectivity of the sodium lactate/glucose separation by nanofiltration. Experimental results were used to get the variation of the observed retentions versus the permeation flux and to evaluate the separation efficiency from the separation factor. In presence of NaCl, both glucose and lactate retentions slightly decrease and remain very close except at low permeation fluxes where the addition of NaCl has more effect on lactate retention than on glucose one. On the contrary, whilst the addition of Na(2)SO(4) has no influence on glucose retention, a strong effect was pointed out on the lactate one, especially for high electrolyte concentrations for which negative retentions were obtained at low permeation fluxes. Then, the separation was much more improved by the addition of Na(2)SO(4) compared to NaCl. A maximum separation factor of 1.9 was obtained with Na(2)SO(4) at 0.25 M added to the glucose (0.1 M)/sodium lactate (0.1 M) solution whereas the separation was impossible without the addition of salt. (C) 2010 Elsevier Ltd. All rights reserved.