When mixing SO with O2 in N2, Ne, or Ar, an end-on complex OS-OO forms in the gas phase and can subsequently be trapped at cryogenic temperatures (2.8-15.0 K). Upon infrared light irradiation, OS-OO converts to SO3 and SO2 + O with the concomitant formation of a rare 1,2,3-dioxathiirane 2-oxide, i.e., cyclic OS([double bond, length as m-dash]O)O. Unexpectedly, the ring-closure of 16OS-18O18O yields a ca. 2 : 1 mixture of cyclic 18OS([double bond, length as m-dash]16O)18O and 16OS([double bond, length as m-dash]18O)18O. The characterization of OS-OO and OS([double bond, length as m-dash]O)O with IR and UV/Vis spectroscopy is supported by high-level ab initio computations.