[en] Dispersed fluorescence spectroscopy and photoelectron spectroscopy have been employed to study vibronic coupling in excited neutral and ionic states of carbon dioxide. The dispersed fluorescence excitation spectra of the CO₂⁺B-bar (001)²Σ_g⁺->X-bar (001)²Π_u and the B-bar (002)²Σ_u⁺->X-bar (002)²Π_g transitions have revealed new structure. Some of this structure can be assigned to nsσ_g or nd Rydberg series converging onto the CO₂⁺C-bar (001)²Σ_u⁺ threshold or to an np Rydberg series converging onto the CO₂⁺C-bar (002)²Σ_g⁺ threshold. Autoionisation from Rydberg states belonging to these series results in the formation of ions with one or two quanta of the asymmetric stretching mode being excited. The HeI excited photoelectron spectra of the CO₂⁺B-bar ²Σ_u⁺andC-bar ²Σ_g⁺ states reveal extensive and complex vibrational structure. Transitions involving the non-totally symmetric bending and asymmetric stretching modes have been observed. The occurrence of these normally forbidden transitions is attributed to vibronic interactions