Self-consistent field and correlated wave functions have been computed for the ground and for several low-lying states of CuO. The ground state is X 2Π and the lowest excited state, at ∼8000 cm-1 above X 2Π, is a previously unidentified 2Σ+ state. The separation of these states is compared to that for the similar states of KO and is analyzed in terms of integrals between orbitals of the separated free ions. We consider a classification of the states of the molecule based on states of Cu+ and O- which leads to a division into manifolds of states arising from Cu+ 3d10 and Cu + 3d9 4s1. We predict that the states of the 3d9 4s1 manifold are 10 000-30 000 cm-1 above the ground state and assign the observed A 2Σ+ state at 16 500 cm-1 to this manifold. © 1983 American Institute of Physics.