We present an ab initio, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkaline earth dications Mg2+, Ca2+, Sr2+, Ba2+, and Ru2+. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree-Fock (RHF) and second-order Moller-Plesset perturbation (MP2) levels of theory using the 6-31+G* basis set and relativistic effective core potentials. The affinity of 18c6 for the dications is considerable, ranging from 150-300 kcal mol(-1). The cation-18c6 interaction arises principally from the electrostatic interaction of the dication with the nucleophilic ether backbone and from the polarization of the crown ether by the electric field of the dication. Whereas Ba2+ selectivity is observed for 18c6 in aqueous environments, our calculations clearly show that the crown ether in fact binds Mg2+ most strongly in gas phase. Thus, solvation effects appear to strongly influence cation selectivity. Indeed, Ba2+ selectivity is recovered when we consider the competition of the solvent and 18c6 molecules for the dications using a simple cation exchange reaction.