The geometry of 2-mercaptobenzothiazole (MBT) and its tautomeric form of benzo- thiazole-2-thione are optimized at B3LYP/6-311G** and HF/6-311G** levels, respectively. The crystal structure of benzothiazole-2-thione and its FTIR spectra are also obtained. From the calcu- lated and experimental data, it can be concluded that in gas-phase and solid-state, the real existing form of 2-mercaptobenzothiazole is the thione-form of MBT.