Abstract Thermodynamic equilibrium constants for the H 2 PO 4 − Cl − and OH − Cl − exchange reactions on Dowex 1 × 8 at 25°C have been determined at 0.1 total normality. The standard free energy of exchange was combined with the standard enthalpy of the exchange obtained from calorimetric measurements to obtain the standard entropy of the exchange. The Cl −-form resin shows little affinity for the OH − and H 2PO 4 − ions because in each case there is an unfavourable (i.e. endothermic) enthalpy change. Although there are favourable (i.e. positive) entropy change for the exchanges, ΔG° is in both cases positive because of the dominant enthalpy term.