The catalyzed S-substitution of 2-mercaptobenzimidazole (MBI or RSH) by tributylamine (TBA) was carried out in an alkaline solution of KOH/organic solvent two-phase medium. Tributylamine (TBA) reacts in situ with the organic-phase reactant, alpha-bromo-m-xylene (R'Br) to produce a quaternary ammonium salt ((C4H9)(3)R'NBr or QBr) which is used as the phase transfer catalyst. The reaction is greatly enhanced by adding a small amount of TBA in the presence of KOH. By appropriated, controlling alpha-bromo-m-xylene (limited compound) at low KOH concentration, only the hydrogen atom connecting to the sulfur atom of 2-mercaptobenzimidazole was substituted by alpha-bromo-m-xylene. A kinetic model was developed in conjunction with the mass transfer of the catalyst (QBr) and the active intermediate (RSQ), as well as the reactions in both the aqueous and organic phases. The two-film theory, is used to account for the mass transfer of QBr and RSQ between two phases. The experimental data verify that the mass transfer rate or the species between the two phases is larger than the reaction rate in the organic phase. The organic-phase reaction is the rate-controlling step for the whole two-phase reaction. A pseudo-steady-state hypothesis (PSSH) is employed to simplify the kinetic model. from which a pseudo-steady-state rate law is sufficiently used to describe the experimental data. Effects of the reaction conditions oil [tic reaction rate and the conversion were investigated in detail.