This work deals with thermodynamic description of equilibrium between an ion exchange resin and the solution of low molecular weight electrolyte. The suggested approach allows calculating thermodynamic constant of ion exchange equilibrium. In order to calculate the activity of ion exchange groups in the resin having monoionic form we used published reference data assuming that the activity of resin exchange sites and that of corresponding monomer are equal in the solutions with equal concentrations. For ion exchange resin in mixed form in equilibrium with the mixed solution of corresponding low molecular weight electrolytes the method of calculating each of ionic forms was also suggested. These activities were used to determine the constant of ion exchange equilibrium. (c) 2006 Elsevier B.V. All rights reserved.