2-Mercaptobenzothiazole (C7H5NS2) has been investigated by synchrotron radiation photoelectron spectroscopy (PES) in the gas phase. The valence region, C 1s and S 2p photoemission spectra have been measured. Comparison of the experimental results with theoretical ab initio calculations, performed for two possible tautomeric forms of the molecule, shows that only the thione form is present in the gas phase. Vibrational structure has been clearly observed in the valence photoemission spectra. In the C Is spectrum the seven components due to the inequivalent carbon atoms in the molecule have been assigned on the basis of ab initio calculations. S 2p components have also been assigned. The photoemission results have been compared with previously reported photoemission measurements for 2-mercaptobenzoxazole.