Bis(trifluoromethyl) sulfoxide, CF(3)S(O)CF(3), isolated in noble gas matrixes at low temperatures, isomerizes upon UV irradiation into the sulfenic ester CF(3)SOCF(3). The new species is characterized spectroscopically, and the vibrational assignment is supported by (18)O isotopic labeling experiments and by DFT calculations. The calculated structural parameters of CF(3)SOCF(3) are compared with the calculated and experimental data of the related compounds CF(3)SSCF(3) and CF(3)OOCF(3). In addition, the computed enthalpy differences between the sulfoxide R(2)S=O and sulfenate RSOR structures for R = H, F, CH(3), and CF(3) are included.