A continuous beam of isopropylthio radical (CH3)(2)CHS was generated in the gas phase by the pyrolysis of isopropyl disulfide C3H7SSC3H7. The electronic structure of isopropylthio radical has been investigated by the in situ HeI photoelectron spectroscopy, in combination with density functional theory and G2 calculations. The first PE band at 8.95 eV with vibrational progression of 625 +/- 60 cm(-1) is designated as the X(3)A" ground ionic state of (CH3)(2)CHS+, deriving from ionization of the electron from the SHOMO. Both photoelectron spectroscopy experiment and theoretical calculations provide evidence for the existence of different ionic states of isopropylthio radical for the first time. (c) 2006 Elsevier B.V. All rights reserved.