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Abstract: For the first time, a study has been made of H2S as a reactant gas in positive-ion chemical-ionization mass spectrometry. To this end, the mass spectrum of an H2S plasma was recorded, and the energies and structures of the ions were examined. The most stable structures of the plasma ions H3S+, S2 +, H2S2 +, H3S2 +, [H2S···H3S]+, H2S3 +, H3S4 + and HS n + were obtained by the ab initio G3B3 method, in which the G3 method is extended with B3LYP/6-31G(d). Of the three isomers of H2S3 +, the C 2V structure, with terminal hydrogen atoms, proved to be the lowest energy. The structure of the adduct [H2S···H3S]+ was found to be comparable with that of the water adduct [H2O···H3O]+. An enthalpy of −59.9kJmol−1 was calculated for the reaction H3S+ +H2S→[H2S···H3S]+ and the exothermic reaction enthalpies of the polysulfonium ions, formed according to the equation HS n + +S→HS n+1 +, lie in the range 225–347kJmol−1 for n =1–8. [Copyright &y& Elsevier]