The 1(2)Pi(X(2)Pi), 2(2)Pi, 1(2)Sigma(+), 3(2)Pi, 4(2)Pi, and 2(2)Sigma(+) states of the OCS+ ion were studied using the CASSCF and CASPT2 methods with a contracted ANO basis and 3(2)Pi and 4(2)Pi are characterized as shake-up ionization states. The CASSCF frequency calculations indicate that the 1(2)Pi, 2(2)Pi, 1(2)Sigma(+), and 2(2)Sigma(+) primary ionization states represent minima in the potential energy surfaces (PESs), and accurate CASPT2 adiabatic excitation energy (TO) calculations support assignments of the A, B, and C states to 2(2)Pi, 1(2)Sigma(+), and 2(2)Sigma(+), respectively. The CASSCF frequency calculations indicate that 3(2)Pi and 4(2)Pi represent saddle points in the PESs. We found minima in the PESs of the 5(2)A' and 4(2)A" states (related to 4(2)Pi), and the CASPT2 To values for 5(2)A' and 4(2)A" are between the To values of B(2)Sigma(+) and C(2)Sigma(+). The observed satellite bands between the B and C bands in spectrum can be (partially) attributed to 5(2)A' and 4(2)A". (c) 2005 Elsevier B.V. All rights reserved.