The 1 (2)A(1), 1 B-2(2), and 1 (2)A(2) electronic states of the SO2+ ion have been studied using multiconfiguration second-order perturbation theory (CASPT2) and two contracted atomic natural orbital basis sets, S[6s4p3d1f]/O[5s3p2d1f] (ANO-L) and S[4s3p2d]/O[3s2p1d] (ANO-S), and the three states were considered to correspond to the observed (X) over tilde, (B) over tilde, and (A) over tilde states, respectively, in the previous experimental and theoretical studies. Based on the CASPT2/ANO-L adiabatic excitation energy calculations, the (X) over tilde, (A) over tilde, and (B) over tilde states of SO2+ are assigned to 1 (2)A(1), 1 B-2(2), and 1 (2)A(2), respectively, and our assignments of the (A) over tilde and (B) over tilde states are contrary to the previous assignments ((A) over tilde to (2)A(2) and (B) over tilde to B-2(2)). The CASPT2/ANO-L energetic calculations also indicate that the 1 (2)A(1), 1 B-2(2), and 1 (2)A(2) states are, respectively, the ground, first excited, and second excited states at the ground-state (1 (2)A(1)) geometry of the ion and at the geometry of the ground-state SO2 molecule. Based on the CASPT2/ANO-L results for the geometries, we realize that the experimental geometries (determined by assuming the bond lengths to be the same as the neutral ground state of SO2) were not accurate. The CASPT2/ANO-S calculations for the potential energy curves as functions of the OSO angle confirm that the 1 B-2(2) and 1 (2)A(2) states are the results of the Renner-Teller effect in the degenerate (2)Pi(g) state at the linear geometry, and it is clearly shown that the 1 B-2(2) curve, as the lower component of the Renner splitting, lies below the 1 (2)A(2) curve. The UB3LYP/cc-pVTZ adiabatic excitation energy calculations support the assignments ((A) over tilde to B-2(2) and (B) over tilde to (2)A(2)) based on the CASPT2/ANO-L calculations. (C) 2004 American Institute of Physics.