Photooxidation reactions of carbonyl sulfide (SCO) in the presence of O3 have been studied in laboratory experiments in the gas phase and in solid argon matrices under stratospheric irradiation conditions. The photolysis experiments performed indicate, that in matrices the reaction of O(1D) atoms with SCO providing CO2 + S as primary products is dominant, whereas in the gas phase the reaction O(3P) + SCO ? CO + SO becomes more important due to the rapid collisional quenching of O(1D). In matrices reaction products CO2, SO2, SO3, and SO4 could be identified.