A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to monitor time-resolved infrared absorption of transient species produced upon irradiation at 248 nm of a flowing mixture of CH3 SSCH3 and O2 at 260 K. Two transient bands observed with origins at 1397±1 and 1110±3 cm-1 are tentatively assigned to the antisymmetric CH3 -deformation and OO stretching modes of syn -CH3 SOO, respectively; the observed band contour indicates that the less stable anti -CH3 SOO conformer likely contributes to these absorption bands. A band with an origin at 1071±1 cm-1, observed at a slightly later period, is assigned to the SO stretching mode of CH3 SO, likely produced via secondary reactions of CH3 SOO. These bands fit satisfactorily with vibrational wavenumbers and rotational contours simulated based on rotational parameters of syn -CH3 SOO, anti -CH3 SOO, and CH3 SO predicted with density-functional theories B3LYP/aug-cc-pVTZ and B3P86/aug-cc-pVTZ. Two additional bands near 1170 and 1120 cm-1 observed at a later period are tentatively assigned to CH3 S (O) OSCH3 and CH3 S (O) S (O) CH3, respectively; both species are likely produced from self-reaction of CH3 SOO. The production of SO2 via secondary reactions was also observed and possible reaction mechanism is discussed.