Geometry optimization and harmonic vibrational frequency calculations have been carried out on the low-lying singlet and triplet electronic states of the antimony dioxide anion (SbO2-) employing a variety of ab initio methods. Both large-core and small-core relativistic effective core potentials were used for Sb in these calculations, together with valence basis sets of up to augmented correlation-consistent polarized-valence quintuple-zeta (aug-cc-pV5Z) quality. The ground electronic state of SbO2- is determined to be the X (1)A1 state, with the a (3)B1 state, calculated to be approximately 48 kcal mole(-1) (2.1 eV) higher in energy. Further calculations were performed on the X (2)A1, A (2)B2, and B (2)A2 states of SbO2 with the aim to simulating the photodetachment spectrum of SbO(2) (-). Potential energy functions (PEFs) of the X (1)A1 state of SbO2- and the X (2)A1, A (2)B2, and B (2)A2 states of SbO2 were computed at the complete-active-space self-consistent-field multireference internally contracted configuration interaction level with basis sets of augmented correlation-consistent polarized valence quadruple-zeta quality. Anharmonic vibrational wave functions obtained from these PEFs were used to compute Franck-Condon factors between the X (1)A1 state of SbO2- and the X (2)A1, A (2)B2, and B (2)A2 states of SbO2, which were then used to simulate the photodetachment spectrum of SbO2-, which is yet to be recorded experimentally.