The electronic structure of CO3 is characterized by equation-of-motion and coupled-cluster methods. C-2v and D-3h isomers are considered. Vertical excitation energies, transition dipoles, and the molecular orbital character of the excited states are presented for singlet and triplet manifolds. Ground-state equilibrium structures and frequencies are strongly affected by vibronic interactions with low-lying excited states. At D-3h geometries, the vibronic interactions are enhanced by the Jahn-Teller character of the excited states. The curvature of the potential energy surface and the existence of the D-3h minimum are very sensitive to the correlation treatment and the basis set. The correlation effects are stronger at D-3h, in agreement with a smaller HOMO-LUMO gap.