Results of ab initio calculations are presented for a series of valence levels of C2+ and for the two lowest states of C2 (x 1Σg+and X′ 3Πu). Correlation energies of all the levels are estimated by a new scheme taking into account near-degeneracy effects in the molecule and in the constituent ion pairs. Results for C2 are in close agreement with experiment for equilibrium internuclear distances, vibration frequencies, term energies and dissociation energies. For C2+ the calculations show the ground state to be Σg−4 with relatively low-lying (∼ 0.6 eV) Δg2 and Πu2 levels. The electron-impact measurement for the appearance potential of C2+ is interpreted as C2(3Πu, υ = 1) → C2+(4Σg−, υ = 0). The quartet transition Σg−4 → Σu−4 is predicted to lie at 6000 Å. The major doublet transitions all lie in the infrared.