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Potassium-calcium exchange equilibria was studied on four soil samples from semi-arid regions of Haryana, India, differing in textural composition and cation exchange capacity, It was observed that major portion (about 80%) of normalized exchange isotherms of all the four soils laid above the diagonal indicating the higher preference of K over Ca, The values of standard free energy change of the exchange reaction (Delta G(r)) was highest for soil-4 (CEC 16.0 Cmol (p+)/kg) and lowest for soil-2 (CEC 5.28 Cmol (p+)/kg). The values of different exchange selectivity coefficients (K-G, K-V, and K-KDO) decreased steeply with increase in exchangeable potassium percentage (EPP). The values of different exchange selectivity coefficients were not constant over the entire range of isotherm, suggesting the presence of different sites for K specificity on the exchange complex of these soil samples.