This paper presents experimental observations on the permeation of mixed-salt solutions through commercial and McMaster pore-filled nanofiltration membranes. Two types of mixed-salt solutions are analyzed (common co-ions and common counter-ions). The permeation of solutions with common counter-ions can be explained from theory and has been predicted accurately in previous work. However, the performance of these membranes when common co-ions are present does not directly follow the behavior expected from normal electrostatic and steric exclusion mechanisms. The membranes behave as if the charge has been inverted. Multivalent counter-ion adsorption is believed to play an important role in the inversion of the effective charge of the membrane, and the subsequent anomalous permeation of counter-ions. (C) 2004 Elsevier B.V. All rights reserved.