Department of Chemistry, North Bengal University, Darjeeting-734 430 Manuscript received 8 September 1988, revised 14 February 1989, accepted 18 March 1989 The uptake of \(Coen^{3+}_3\)+ at a particular pH (7.5) by the syntlletic humic acids, one [SyHA(A)] prepared from benzoquinone and ammonium chloride and [SyHA)(B)] prepared from benzoquinone and casein, show almost a linear relationship till the major exchange sites available at this pH are saturated. The exchange reactions, however, at different pH values exhibit three distinct stages of interactions. The release Isotherms with monovalent cations are more or less of Langmuir type for Coen3 -SyHA(A) system and S-shaped for Coen3 -SyHA(B) complex. With both the systems the orders of reactivities have been evaluated. By using Kielland s equation, the thermodynamic equilibrium constants and standard Gibbs free energy changes for exchange reactions involving bivalent cations in respect of Coen3- SyHA(A) complex have been calculated. It is observed that the log of corrected selectivity coefficients or thermodynamic equilibrium constants when plotted against the hydrated ionic radii and the reciprocals of the Debye- Huckel parameter ao indicate that the relative affinities of the counter ions for exchanger substrates are much better correlated with ao rather than with ionic radii particularly for monovalent cations.