In comparison to well-established aqueous processes, non-aqueous processes are characterized by the increase in the number of solute solvent membrane interactions, which play a determining role in the understanding of solvent flux and solute rejection. Straight extension of separation mechanisms, typical of aqueous environments, to non-aqueous systems is complex, due to the significant differences in the structures and properties of the solvents. In such complex systems, competition between solute membrane and solvent membrane affinities may become critical. The aim of this work is to study the effect of the solute solvent competition, in terms of relative affinity with the membrane, on the nanofiltration performance in Organic Solvent Nanofiltration. Permeation of single salts and acids (NaCl, KCl, LiCl, Nal, NaF, HCl and trifluoroacetic acid, TFA-H) in water and in organic/water mixtures through hydrophilic ceramic membranes (TiO2/Al2O3) was investigated. The effect of charge was found to affect the ion retention in water, while it was found to be less significant in the presence of organic solvents. Competition between solute and solvent in terms of preferential solvation was found to affect nanofiltration and ultrafiltration of the charged solutes. The effects of pore dimension, nature of the organic solvent and nature of anion/cation for different salts and acids are presented and discussed. Afterwards, rejections of one small organic molecule, Npys, and one model peptide, PEP1, through the same hydrophilic ceramic membranes, were studied as a function of solvent composition (%v ACN/water) and ion content (Na+, H+, Cl-, and TFA(-)). The roles of preferential solvation and association of chaotropic/kosmotropic ions are introduced to explain the salt effects on the rejection of the organic solutes. (C) 2013 Elsevier B.V. All rights reserved.