Ion-exchange equilibrium between Cs and H on a sulfonated polystyrene resin of Dowex 50W with 8:; crosslinking has been studied over a temperature range of 25" to 60°C, using I3'Cs as radioactive tracer; the various experimental data related to the equilibrium experiments were examined in detail by several different methods; a new treatment to take into account the water uptake in the exchange system Cs-H was also applied with the aid of a radioactive tracer technique. The thermodynamic equilibrium constant K g determined by the present study compares well with the values calculated indirectly from experimental data found by other investigators. Data are also given for the heats, free energies and entropies of the exchange reaction involving Dowex 50W. Fairly good results have been obtained on the exchange of Cs and H.