The influence of ionic strength on the peptide fractionation process by means of polyethersulfone membranes was studied in this work. In the case of pH conditions in which electrostatic interactions between the membrane and the peptides are not significant, the addition of salt increased the transmission of all peptides as a result of a reduction in their effective size. However, when the charge effect was the main mechanism responsible for the fractionation process, the combination between the reduction of both the peptide radius and the charge density of the membrane affected the transmission of cationic and anionic species differently. While the transmission of negatively charged peptides increased considerably, the transmission of positive solutes scarcely changed or even decreased. Despite the good separation factors between charged peptides observed under low ionic strength conditions, the selectivity of the process decreased when working at a high salt content. Furthermore, the separation factor between bioactive and non-bioactive peptides decreased slightly under high ionic strength conditions. The results obtained in this work demonstrate that peptides from milk protein can be fractionated at different pH by modifying the solution ionic strength. (C) 2013 Elsevier B.V. All rights reserved.