This study investigates the sorption of synthetic organic compounds, such as benzene sulfonate, naphthalene mono- and disulfonates, onto ion exchangers. These organic compounds exist as aromatic anions in aqueous solutions within a wide range of pH and they are referred to as hydrophobic ionizable organic compounds (HIOCs). Characteristically, the HIOCs contain two primary constituents: non-polar aromatic head groups and ionic charges. Sorption of the HIOCs onto ion exchangers essentially follows an ion exchange stoichiometry; however, ion exchange selectivity is greatly governed by the size of the non-polar head group and ionic charge. The favorable sorption of the HIOCs is unique because they are all endothermic and entropy-driven. The ion exchange selectivity in favor of the HIOCs can be reduced and reversed in cosolvents. Chloride and sulfate are commonly encountered competing ions presented in water systems. The valence of the competing ions influences sorption of the HIOCs onto ion exchangers. An equilibrium model can be used to predict the valence effect of the competing ions. (C) 2004 Elsevier B.V. All rights reserved.