Concentration equilibrium quotients, Le., selectivity coefficients, have been measured a t 30" for the following cation-exchange systems and solvents using Dowex 50 X8: Li-H, and Li-Ya in methanol-water mixtures; Sa-H in methanol-ethanol and in ethanol-water mixtures; Sa-H as a function of resin composition in pure ethanol; and Li-8 as a function of resin composition in pure methanol. External solutions were maintained a t 0.1 m ionic strength. Alterations in the degree of ion solvation and ionpair formation are believed to account for the generally enhanced metal selectivities in alcoholic s) stems as compared with similar aqueous systems. A maximum in the selectivity for the "metallic" ion was noted in every "metal"-hydrogen exchange in methanol-water and ethanol-water a t external solution concentrations of from about 15 to 30 mole yo of water. An interpretation is offered based on the effect of the medium on ionic activity coefficients, especially that of the proton. The equilibrium quotients for the Na-H and K-H exchanges in methanol-water are corrected for activity effects in the external solution. The absence of maxima in the Na-H system in methanol-ethanol and in the Ka-Li system in methanol-water is discussed.