The selective HDS of FCC gasoline is a sensitive option for reducing sulfur content in commercial gasoline. For such applications, a minimum activity of the catalyst toward olefin hydrogenation is required to preserve the high octane number of the feedstock. The conversion of a model FCC olefin (2,3-dimethyl-2-ene: 23BMB2N) under close HDS conditions was investigated over unsupported molybdenum sulfide-based catalysts promoted by nickel or cobalt. The effect of the partial pressure of H2S, as produced during HDS process, on hydrogenation activity of molybdenum-based catalysts was specially studied. The results reveal and confirm the highest promoting effect of nickel compared to cobalt in the hydrogenation of the olefin on unsupported transition metal sulfides (TMS) which is consistent with the fact that CoMo/Al2O3 catalyst is more selective than the NiMo/Al2O3 catalyst for the HDS of FCC gasoline. However, unsupported NiMo catalyst was also the most sensitive towards H2S for olefin hydrogenation. Therefore, for high sulfur content, the selectivity difference between CoMo and NiMo catalysts might be reduced. (C) 2008 Elsevier B.V. All rights reserved.