A continuous flowing CH3SS radical beam is produced in situ by pyrolysis of CH3SSCH3 at 215(+/-0.5) degreesC. An obvious and complete photoelectron (PE) spectrum of the CH3SS radical is recorded in situ for the first time. Five sharp peaks at 8.63, 9.36, 9.94, 10.29, 10.72 eV and one broader band at 11.82 eV are observed in the PE spectrum below 12 eV. The first peak with the lowest ionization energy, 8.63 eV, is attributed to removal of an electron from the highest occupied molecular orbital (HOMO) 5a" of the CH3SS radical, corresponding to the CH3SS+(X (1)A')<-- CH3SS(X(2)A") ionization. The vibrational spacing 600 +/- 60 cm(-1) on the first peak corresponds to the S-S stretch mode excited in the CH3SS+ cation upon photoionization process. The sharp peaks at 9.36 and 9.94 eV come from removal of the electron from the 16a' orbital, leading to (3)A" and (1)A(') ionic states of the CH3SS radical, and the sharp peaks at 10.29 and 10.72 eV should be the result of removal of the electron from the 15a' orbital of the CH3SS radical. To assign the photoelectron spectroscopy (PES) bands of the CH3SS radical, both GAUSSIAN2 (G2) and improved density functional theory (DFT) calculations on different ionic states of the CH3SS radical are also performed. The PES experimental ionization energies of the CH3SS radical agree with those calculated with G2 and DFT. (C) 2001 American Institute of Physics.