This investigation attempts to examine the hydrophobic interactions of ion-exchanger's ligand on protein adsorption at mild to high ionic strengths in solution. Adsorption isotherms and enthalpies of lysozyme and myoglobin on cation exchanger resin (CM-Sepharose, SP-Sepharose and SOURCE 30S) were measured at selected ionic strengths of phosphate buffer. Binding enthalpies showed that adsorption occurs mainly by hydrophobic interaction at high ionic strength. The investigation found the binding affinities and enthalpies were ligand-dependent and CM-Sepharose contains the most hydrophobic ligand among those studied. Results of this study provide further insight into the hydrophobic effect of ion-exchange resin on chromatography applications for protein purification under high ionic strength conditions. (c) 2006 Elsevier B.V. All rights reserved.