The (1:1) Na+ equilibrium constants, K-e1, of macrocyclic ethers of crown-4, crown-5 and crown-6 were determined with a Na+ ISE in the presence of a second cation e.g. Li+, K+ and Ca2+ in dioxane/water (50/50). We estimated the (1:1)equilibrium constants, K-e2 of the macrocyclic ethers with Li+, K+ and Ca2+ by this way. The binding selectivity of a macrocyclic ether between two cations was estimated in the same binary solvent mixture where the water hydration role is diminished. Results showed clearly the effect of macrocyclic size and cation radii in a solution.