The Hogfeldt three-parameter model has been applied to nitrate-chloride ion exchange in diluted and in concentrated solutions. The three parameters of the model have been calculated and their variation with the water activity of the solution, the crosslinking degree of the resin, and the nature of the co-ion (Li+, Na+, K+, and NH4+) has been determined. For a given resin and a given co-ion the results show that the Hogfeldt model can be applied to ion-exchange data in concentrated solutions only if the variation of the parameters with the water activity of the solution is taken into account.