Department of Chemistry, University of North Bengal, Dist. Darjeeling, West Bengal-784 430 Manuscript received 5 May 1981, revised 25 September 1981, accepted 7 January 1982 The exchange Isotherms of Co(tn)\(_8^{8+}\) from Na-co(tn)3 IRC-50 by monovalent ions in aqueous and 50% ethanol-water (v/v) media are of the S-type indicative of cooperative sorption. In aqueous solution the desorption efficiency of the ions Is in the order : NH4>LI>Na>K>Rb>Cs while In 50% ethanol-water the sequence Is Na>K>Li> Rb>Cs. The extent or release or Co(tn)\(_2^{2+}\)from the resin matrix by each ion is much higher in 50% ethanol-water medium than In pure aqueous solution. It has been suggested that site binding of the ions with the resin followed by release of water molecules from the solvation shells of the participating species takes place. The plots of log(selectivity coefficient) against both the hydrated Ionic radius and the reciprocal of the Debye Hṻckei ion-size parameter aº In aqueous medium are linear. This implies that both the parameters may be used to correlate the relative affinities or the mono· valent Ions for the resin surface.