The singlet excited states and doublet ionized/electron-attached states of o-benzoquinone (o-BQ) were studied by the symmetry adapted cluster (SAC)/ SAC-configuration interaction (SAC-CI) method, and the corresponding spectral bands were assigned. The calculated transition energies reasonably reproduced the experimental spectra with the mean error of about 0.2 eV, except for the 1 (1)A(1) states, whose disagreement may be attributed to involvement of the shoulder peak of this state in the intense peak at approximately 6.2 eV. For the singlet states, the lowest four excited states were assigned to n(+)-pi(+)* (B-1(1)), n(-)-pi(+)* ((1)A(2)), pi(-)-pi(+)* (B-1(2)), and pi(+)-pi(+)* ((1)A(1)) in order of increasing energy, and the intense band at approximately 6.2 eV in the experimental spectra was assigned to the B-1(2) state in our calculations. For the cation doublet states, the lowest four states were assigned to n(+) ((2)A(1)), pi(-) ((2)A(2)), n(-) (B-2(2)), and pi(+) (B-2(1)) in order of increasing energy. Shake-up ionized states appeared in the energy region higher than 16 eV. For the anion doublet states, the ground state was B-2(1), and five valence excited states were calculated within 4.0 eV above the anion ground state. The adiabatic electron affinity was calculated to be 1.63 eV, which is in very good agreement with the corresponding experimental value (1.62 eV). The use of Koopmans' theorem does not reproduce this energy order for either the singlet or the doublet states. We showed that, as in the case of p-BQ (J. Phys. Chem. A 2002, 106, 3838), electron correlation is essential in the description of the excited states of o-BQ.