Abstract The most stable conformation of ion-molecule complexes involving a CO molecule were surveyed by the use of Hartree-Fock (HF) MO and third-order Moller-Plesset perturbation (MP3) methods with a 6–31G* basis set ion = H + , Li + , Na + , K + , Bc 2+ , Mg 2+ , and Ca 2+ . The MP3 level of theory reveals the ion-CO conformation in which the ion bonds to a carbon atom of CO to be the most stable; these MP3 results are contrary to the HF ones. Binding energies of ion-molecule complexes involving CO and N 2 were computed; MP3 energies are in good agreement with the experimental ones. The computed binding energies of cation-N 2 are about one-third of cation-NH 3 due to the absence of dipole moment and the smaller polarizability of N 2 . The decrease in binding energy in cation-CO and -N 2 complexes, with increasing cation size, is mainly caused by the decrease of the electrostatic and polarization stabilizations.