By use of exchange isotherms determined at different temperatures, the chemical potentials and partial molar heats and entropies of the sodium-hydrogen ion-exchange reaction on a sulphonated polystyrene exchanger are evaluated. These show that both species are almost completely ionized in the exchanger, and very similar to their states in aqueous solution. In the high concentrations obtainable in the exchanger, however, there is some cation-cation interaction, which is practically linear with composition. A similar treatment of the barium-hydrogen system indicates that there is some interaction between the barium ion and the resinate structure.