Photoionization-efficiency (PIE) spectra in the wavelength range 108-144 nm were measured for CH3SCl generated from the reaction system Cl/Cl-2/CH3SH(CH3SSCH3) in a discharge-flow reactor coupled to a photoionization mass spectrometer employing a synchrotron as the source of radiation. According to the PIE spectrum of CH3SCl thus obtained, the ionization energy (IE) is (9.147+/-0.007) eV. Based on GAUSSIAN-2 calculations, the observed ionization of CH3SCl near the threshold region is likely to form from singlet CH3SCl to doublet CH3SCl+; the calculated IE 9.140 eV agrees well with the experimental value. Two vibrational frequencies of singlet CH3SCl and one of doublet CH3SCl+ were found to be (244+/-60) cm(-1), (513+/-60) cm(-1), and (631+/-60) cm(-1), respectively.