The study on thermodynamics of ion exchange equilibrium for uni-univalent H+ / Na+, H+ / K+ reaction systems were carried out using strongly acidic cation exchange resin Indion-225. For both the uni-univalent ion exchange reaction systems, the equilibrium constant K’ were calculated by using Bonner et. al equation by taking into account the mole fraction of ions in the resin phase. The equilibrium constant values calculated for uni-univalent cation exchange reaction systems were observed to increase with rise in temperature, indicating the endothermic exchange reactions having enthalpy values of 27.4 and 13.2 kJ /mol, respectively. On the basis of enthalpy values, the preferential selectivity of ion exchange resins in H+ form for different univalent ions in aqueous solution is predicted.