Sulfur trioxide (SO3) isolated in solid argon at 12 K was irradiated with light at 193 nm from an ArF excimer laser. Recombination of photofragments O and SO2 produces OSOO that absorbs at 1229.6, 1041.3, and 597.6 cm-1. The assignments are based on observed 34S- and 18O-isotopic shifts. Theoretical calculations using the B-P86 and the B3-LYP density functional methods were carried out for five isomers of SO3; energies and vibrational wave numbers were predicted for each one. Observed line positions, infrared intensities, and isotopic shifts fit well with those predicted for cis-OSOO at the B-P86 level. Further irradiation of the matrix sample with emission at 248 nm from a KrF laser bleached OSOO and enhanced absorption lines of SO2 and, to a lesser extent, of SO3. The mechanism of formation of OSOO in a matrix cage is discussed.