Ab initio studies of the binding of Na ⁺ with a series of 4-substituted monobenzo 15-crown-5 ligands are reported and compared with experimental data. The substituent effect on the cation binding is small and almost negligible for methyl and amino groups but a difference of ca. 12% in the binding energy is found between the 4-methyl and 4-nitro derivatives. The substituent effect is related to the basicity of the aromatic ether oxygens as demonstrated by the correlation found by plotting binding energy (ΔE) vs. average charge of aromatic ether oxygens. A Hammett correlation is found by plotting ΔE vs. σ _p ^- +σ _m . Ab initio studies of the binding of Na ⁺ and K ⁺ with 3n-crown-n (n=4–7) ligands are reported and the effect of a change in the ring size on the cation selectivity is evaluated. Both Na ⁺ and K ⁺ bind to 18-crown-6 ether more strongly than any other 3n-crown-n ether studied here. The ring-size effect on the cation selectivity is found to depend not only on the hole-size concept, but also on the number of available donor atoms that can effectively participate.