We describe the first preparation of the long-sought parent oxathiirane from sulfine through photochemical rearrangement with light at lambda = 313 +/- 10 nm in an Ar matrix at 11 K. Oxathiirane was characterized by the extraordinarily good agreement of experimentally measured and (unscaled) CCSD(T)/cc-pVTZ computed vibrational frequencies for both the perhydrogenated and perdeuterated species. The title molecule is ca. 10 kcal mol(-1) less stable than sulfine, in marked contrast to the isomer energy difference of dioxirane vs carbonyl oxide (ca. -25 kcal mol(-1)). This is due to the strong positive polarization (blue potential) versus the highly electronegative oxygen atom (red). The stability ordering and the relative energy differences of carbonyl versus thiocarbonyl groups underline the likely role oxathiiranes play in sulfur transfer reactions.