Retention properties of a nanofiltration ceramic membrane were investigated with single polyethyleneglycol (PEG) solutions and mixed PEG/inorganic electrolyte solutions. The rejection coefficient of PEGs was found to decrease in the presence of ions. It was shown that the effect of ions on the retention of neutral solutes increases with the electrolyte concentration. This phenomenon was ascribed to the partial dehydration of PEG molecules induced by the surrounding ions. This argument was confirmed by using various electrolytes (KCl, LiCl and MgCl2). It was found that the lowering of the PEG rejection coefficients follows the Hofmeister series, i.e. Mg2+ > Li+ > K+. Experimental data were used to compute the resulting decrease in the Stokes radius of PEG molecules in the presence of the various electrolytes. (c) 2007 Elsevier B.V. All rights reserved.