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(O)O. Unexpectedly, the ring-closure of ¹⁶OS–¹⁸O¹⁸O yields a ca. 2 : 1 mixture of cyclic ¹⁸OS(¹⁶O)¹⁸O and ¹⁶OS(¹⁸O)¹⁸O. The characterization of OS–OO and OS(O)O with IR and UV/Vis spectroscopy is supported by high-level ab initio computations.