The effect of various salts on the viscosity, and by implication structure, of water in polymeric membrane pores of radius similar to1.69 nm and low charge density has been studied. Permeation of pure water and various electrolyte solutions was analyzed using the Hagen-Poiseuille equation expressed in a ratio form to exclude membrane-specific quantities such as pore radius and length. The analysis produced viscosity ratios of electrolyte to pure water inside the membrane pores. Comparing the viscosity ratios inside the pores with their bulk counterparts showed that confinement significantly increased the sensitivity of water structure to the presence of ions. It has been found that, in relative terms in the pores, Cl- was a strong structure breaker, K+ was a moderately strong structure breaker, Na+ was a weak structure breaker, SO42- was a weak structure maker, and Mg2+ was a strong structure maker. Predictive modeling of membrane separation performance would benefit from such effects being taken into account in cases where the pore ion concentrations may be high. (C) 2003 Elsevier Inc. All rights reserved.