Exchange reactions have been carried out between varying amounts of H-resin and varying con­centrations of montmorilionibe days saturated With Na+. K+, NH4+, Ca2+, Mg2+ and Ba2+ lons in aqueous medium end the selectivity coefficients have been calculated. The selectivity coefficients demean with the increase in concentration of the play suspension. The decrease is most prominent in the case of K-saturated clay. The selectivity coefficient probably depends on the relative affinities of the ions towards the solid exchangers. The decrease in selectivity coefficient with day concentration is attributed to the overlapping of electrical double layer and subsequent immobillsation of leas. The isotherms obtained with the varying amounts of II-resin are somewhat reversed in shape to ordinary adsorption isotherm To asertain all the role of ionic hydration the experiments have also been carried out in 50% aqueous ethanolic medium and the decrease in selectivity coefficient is found to be more prominent : The somewhat low values of the selectivity coefficient of Na+ and K+ ions in ethanolic medium are probably due to the low dielectric constant of the medium .