The structure and energetics of the isomeric H-bonded complexes OC⋅⋅⋅HF and CO⋅⋅⋅HF have been investigated by ab initio molecular orbital theory and by natural bond orbital analysis. Only with the inclusion of electron correlation is a significant preference for the experimentally observed OC⋅⋅⋅HF isomer found. The large effect of correlation upon the relative stability of the two isomers is apparently entirely an electrostatic effect caused by the correlation-induced sign reversal of the dipole moment of CO. Nevertheless, a molecular multipole expansion is found inadequate to account for the principal features of these H-bonded complexes and their relative stability. Contrary to a recent study, we find that ‘‘charge transfer’’ effects are highly significant contributions to the binding in these complexes. The differences in stability of OC⋅⋅⋅HF and CO⋅⋅⋅HF are attributed primarily to differences in the interaction of carbon and oxygen lone pairs of CO donating into the unfilled antibond on HF, i.e., to differences in nC→σHF* and nO→σHF* matrix elements.