AbstractPhotooxidation of carbonyl sulfide (SCO) in the presence of ozone has been studied in solid argon matrices under stratospheric irradiation conditions. The matrix experiments allowed the isolation and identification of SCO.O3 1:1 aggregates. Selective photolysis of O3 yielded O(1D) atoms and subsequent reaction with SCO resulted in the primary formation of CO2 + S. The sulfur atoms generated this way are oxidized to SO2 by O2, which has been formed during photofragmentation of O3. In further steps SO2 reacted with diffusing O atoms to SO3 and SO4. Experiments with isotopically labeled precursors confirmed the conclusions that the photolysis of 18O3.SCO hetero-complexes leads to S18O2, S18O3, and 16OC18O formation.