The aim of this work was to investigate NF as a purification step, i.e. sugar removal, in the production process of lactic acid from sodium lactate fermentation broth. Experiments were carried out with the Desal 5 DK membrane and solutions of increasing complexity, i.e. single-solute solutions of sodium lactate and glucose and mixed-solute solutions containing both solutes. Concentrations close to those of a fermentation broth were chosen. Experimental results were used to get the variations of the intrinsic retention versus the permeation flux in order to achieve comparisons without the interference of concentration polarization. Quite distinct retentions were obtained for glucose and sodium lactate in single-solute solutions so that the purification was expected to be feasible. However, it is pointed out that glucose retention is significantly lower in mixed-solute solutions, i.e. when sodium lactate is present. This decrease is such that the retentions of both solutes become comparable so that any purification is unachievable. Experiments were also performed with a salt of a different nature (mineral salt, NaCl). Again, it was found that the presence of NaCl tends to decrease glucose retention. Moreover, the phenomenon is shown to be related to the salt concentration in both cases, i.e. with sodium lactate and NaCl. Some possible explanations of this effect are provided in this paper. Further investigations are still in progress to improve the knowledge of the mechanisms involved. (c) 2005 Elsevier B.V. All rights reserved.