The interaction of guanine and adenine with ions of groups Ia, Ib, IIa, and IIb were studied at the Hartree-Fock and second-order Moller-Plesset levels employing all-electron (AE) and pseudopotential treatments. Christiansen’s average relativistic effective pseudopotentials (AREP) were used for all the ions with the exception of Li+. AE and AREP treatments were tested for the Na+, K+, Mg2+, and Ca2+ complexes; very good agreement between the results suggests that pseudopotentials can be used with confidence also for other cations. Intermolecular X-N7 distances for complexes containing adenine are shorter than those for complexes containing guanine. The stabilization energies for guanine ... X complexes are larger than those of adenine ... X complexes. Relativistic effects are most pronounced for Au+ and Hg2+ ions.