The main purpose of the present theoretical work was to study predissociation mechanism of the C (2)Sigma(g)(+) state of the CO(2)(+) 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 (4)B(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(infinityv), 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 (4)B(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 CO(2)(+) (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).