In this paper, an experimental study was carried out in order to investigate the osmotic transport of methanol-water electrolyte solutions through a Nation membrane. The experimental data indicated that the Nafion membrane showed the typical anomalous osmotic behavior of charged membranes. The influence of some relevant parameters, such as electrolyte concentration difference, weight fraction of methanol on solution, and nature of cation was considered. The results showed that the osmotic volume flow was decreased with the presence of methanol on solvent, but did not alter the anomalous osmotic behavior of the membrane. (C) 2003 Elsevier Science (USA). All rights reserved.