Sulfur dioxide (OSO) isolated in solid argon at 13 K was irradiated with light at 193 nm from an ArF excimer laser. Weak absorption lines at 1006.1, 1004.7, and 739.9 cm-1 observed after photolysis are assigned to sulfur superoxide (SOO); the doublet near ∼1005 cm-1 is due to matrix site splitting. The assignments are based on results from 18O-isotopic experiments. Calculations using the B-P86 and the B3-LYP density-functional methods were made for three isomers of OSO; energies, structures, vibrational wave numbers, and infrared intensities were predicted for each species. Although observed line positions. are nearly identical to those predicted with theory for cyclic-SO2, experimental relative IR intensities and 18O-isotopic shifts agree with those predicted for SOO. The mechanism of formation of SOO in a matrix cage is discussed.