The equilibrium and kinetic behaviour of weakly acidic resin (WAR) with alkali and weakly basic resin (WBR) with acids have been studied by many research teams. But little work is reported in literature regarding equilibrium and kinetic behaviour of mixture of WAR and WBR.In the present study, detailed analysis of equilibrium and kinetic behaviour of mixed resins with sodium chloride has been undertaken. From equilibrium studies, no adsorption is observed taking place, when salt solution is brought in contact with WAR or WBR individually; however adsorption is observed taking place, when salt solution is brought in contact with the mixture of WAR and WBR. The salt uptake depends on the concentration of salt solution, the amounts of individual ion-exchange resins as well as their proportion.Mixture of resins and salt solution has three phases interacting with each other and a number of resistances can influence salt uptake rate either individually or in combination. A series of possible resistances to the salt uptake are identified, namely, diffusion across films surrounding the resin particles, diffusion inside the resin particles, and dissociation of carboxylic group (-COOH) in WAR.To minimize the mathematical complexity, limiting case approach is adopted in this study. The underlying assumption in this approach is that resistance to the entire process lies in that limiting step.It is finally concluded that generation of H+ ions (by interaction of WAR particles and salt solution) followed by adsorption of acid by WBR governs the salt uptake process in mixed resin systems. (C) 2004 Elsevier Ltd. All rights reserved.