Photoionization efficiency (PIE) spectra of CH3SO, CH3SOH, and CH3SS(O)CH3 were measured in the spectral range 110-150 nm by means of a discharge flow system coupled to a photoionization mass spectrometer that employs a synchrotron radiation source. These species were generated by reacting O atoms with CH3SSCH3 in the flow tube; a microwave discharge was employed to produce O atoms from O2. Alternatively, C2H5SCH3 replaced CH3SSCH3 to form CH3SO, CH3SOH, and other products. Ionization thresholds of 8.99 ± 0.02, 8.67 ± 0.03, and 8.82 ± 0.05 eV were determined for the first time for CH3SO, CH3SOH, and CH3SS(O)CH3, respectively. The former two values agree well with theoretical calculations with the G2 method. The latter value fits satisfactorily with those predicted for CH3SS(O)CH3 with the G2MP2 method; the G2 method was not employed in this case due to the numerous atoms in this molecule. lonization thresholds of CH3S and CH3S2 were also determined to be 9.27 ± 0.03 and 8.62 ± 0.05 eV, respectively, in agreement with recent experimental reports.