In the present work, we mainly study dissociation of the C 2 B 1 , D 2 A 1 , and E 2 B 2 states of the SO 2 1 ion using the complete active-space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods. We first performed CASPT2 potential energy curve (PEC) calculations for S-and O-loss dissociation from the X, A, B, C, D, and E primarily ionization states and many quartet states. For studying S-loss predissociation of the C, D, and E states by the quartet states to the first, second, and third S-loss dissociation limits, the CASSCF minimum energy crossing point (MECP) calculations for the doublet/quartet state pairs were performed, and then the CASPT2 energies and CASSCF spin-orbit couplings were calculated at the MECPs. Our calculations predict eight S-loss predissociation processes (via MECPs and transition states) for the C, D, and E states and the energetics for these processes are reported. This study indicates that the C and D states can adiabatically dissociate to the first O-loss dissociation limit. Our calculations (PEC and MECP) predict a predissociation process for the E state to the first O-loss limit. Our calculations also predict that the E 2 B 2 state could dissociate to the first S-and O-loss limits via the A 2 B 2 / E 2 B 2 transition. On the basis of the 13 predicted processes, we discussed the S-and O-loss dissociation mechanisms of the C, D, and E states proposed in the previous experimental studies.