When a high-frequency discharged CS2:O-2:Ar sample was condensed at 4 K, new absorptions at 1003.1 and 567.7 cm(-1) were produced. Isotopic substitution ((CS2)-C-13/O-2, (CS2)-S-34/O-2, CS2/O-18(2), and mixtures) shows that these new absorptions are due to S-O and S-S stretching vibrations, which involve two inequivalent S atoms and one O atom. The photosensitive behavior and the agreement with frequencies and isotopic frequency ratios from quantum chemical calculations substantiate assignment of these absorptions to the SSO- anion, which was predicted to have a bent structure with elongated S-S and S-O bond lengths relative to those of S2O.