Ab initio studies of the aluminum monocarbonyl species AlCO and AlOC have been performed to predict the geometries, fragmentation energies, and harmonic vibrational frequencies. Both species were optimized at the self-consistent field, configuration interaction, and coupled-cluster levels of theory with large basis sets. At the highest level of theory, AlCO was found to be 22 kcal/mol more stable than the isocarbonyl, AlOC. Al–CO was found to have a dissociation energy of 9 kcal/mol, with no barrier to dissociation to Al and CO fragments. Al–OC was found to have a dissociation energy of −13 kcal/mol with an energy barrier to dissociation of less than 5 kcal/mol. The dipole moment of AlCO is found to be small (around 0.1 D), while that of AlOC is significantly larger (around 2.8 D). The C–O harmonic vibrational frequencies were evaluated at all levels of theory. For AlCO at the highest level of theory, the C–O frequency was 1914 cm−1 compared to the experimental value of 1868 cm−1, a 2.5% difference which may be attributed largely to anharmonic effects. The C–O frequency for AlOC is remarkably close to a tentative and disputed experimental spectral feature.