Bis(trifluoromethyl) sulfoxide, CF3S(O)CF3, isolated in noble gas matrixes at low temperatures, isomerizes upon UV irradiation into the sulfenic ester CF3SOCF3. The new species is characterized spectroscopically, and the vibrational assignment is supported by O-18 isotopic labeling experiments and by DFT calculations. The calculated structural parameters of CF3SOCF3 are compared with the calculated and experimental data of the related compounds CF3SSCF3 and CF3OOCF3. In addition, the computed enthalpy differences between the sulfoxide R2S=O and sulfenate RSOR structures for R = H, F, CH3, and CF3 are included.