The main purpose of the present theoretical work was to study predissociation mechanism of the C (2)Sigma(+)(g) state of the CO2+ ion using the CAS methods. Since the X (2)Pi(g), A (2)Pi(u), B (2)Sigma(+)(u), 1 (4)Sigma(-)(g), and 1 (4)Pi(u) states are involved in the predissociation, we also studied these five states. The CASPT2 calculations indicate that Renner-Teller splitting in 1 (4)Pi(u) leads to two C-2v states, 1 (4)A(1) and 1 B-4(1). For the X (2)Pi(g), A (2)Pi(u), B (2)Sigma(+)(u), and C (2)Sigma(+)(g) states, the CASPT2 T-0 values and geometries are in good agreement with experiment. The CASPT2 calculations for the O-loss dissociation potential energy curves indicate that the 1 (4)Sigma(-)(g), X (2)Pi(g), 1 (4)Pi(u), A (2)Pi(u), B (2)Sigma(+)(u), and C (2)Sigma(+)(g) states correlate with the first, second, second, third, third, and fourth dissociation limits, respectively. The CASSCF minimum energy crossing point (MECP) calculations (in the C-infinity v, C-s, and C-2v symmetries) were performed for selected state/state pairs, and the spin-orbit couplings were calculated at the MECPs, All the MECPs (including the C (2)Sigma(+)(g)/1 (4)Pi(u) (1 B-4(1)) MECP), involved in the proposed predissociation mechanism of Praet et al. (J. Chem. Phys. 1982, 77, 4611-4618), were found and the calculated spin-orbit couplings at these MECPs are not small. Our calculations support the mechanism of Praet et al. and indicate that an energy value of 8.9 eV from CO2+ (X (2)Pi(g)) is needed. The C (2)Sigma(+)(g) state in the previous [1 + 1] photodissociation experiments (J. Chem. Phys. 2008, 128, 164308.) could predissociate through the mechanism of Praet et al. since the two-photon energy was around 8.9 eV, while the C (2)Sigma(+)(g) state in the previous VUV-PFI-PE experiments (J. Chem. Phys. 2003, 118, 149-163) would predissociate through another mechanism via A (2)Pi(u).