Irradiation of an Ar matrix sample containing O3 and CS 2 with a KrF excimer laser at 248 nm yielded new lines at 1402.1 (1404.7), 1056.2 (1052.7), and 622.3 (620.5) cm-1; numbers in parentheses correspond to species in a minor matrix site. Secondary photolysis at 308 nm diminished these lines and produced mainly OCS and SO2. Annealing of this matrix to 30 K yielded a second set of new lines at 1824.7 and 617.8 cm-1. The first set of lines are assigned to CS stretching, O-S stretching, and S-C stretching modes of carbon disulfide S-oxide, OSCS; and the second set of lines are assigned to CO stretching and OCS bending modes of dithiiranone, O(CS2), respectively, based on results of 34S- and 18O-isotopic experiments and quantum-chemical calculations. These calculations using density-functional theory (B3LYP/aug-cc-pVTZ) predict four stable isomers of OCS2: O(CS 2), SSCO, OSCS, and SOCS, listed in order of increasing energy. According to calculations, O(CS2) has a cyclic CS2 moiety and is the most stable isomer of OCS2. OSCS is planar, with bond angles OSC≅111.9° and SCS≅177.3°; it is less stable than SSCO and O(CS2) by ∼102 and 154 kJ mol-1, respectively, and more stable than SOCS by ∼26 kJ mol-1. Calculated vibrational wave numbers, IR intensities, 34S- and 18O-isotopic shifts for OSCS and O(CS2) fit satisfactorily with experimental results.