The S-alkylation (substitution on sulfur atom) of 2-mercaptobenzimidazole (MBI) by ally] bromide (RBr) catalyzed by quaternary ammonium salts was successfully carried out in an aqueous solution of KOH/organic solvent two-phase medium. No product was obtained from N-alkylation (substitution on the nitrogen atom) during or after the reaction period using a limited quantity of ally] bromide at a low alkaline concentration of KOH. The reaction is greatly enhanced by adding a small amount of potassium hydroxide. The conversion of allyl bromide for the substitution of MBI either by cetyltrimethylammonium bromide (CTMAB), benzyltriethylammonium chloride (BTEAC), polyethylene glycol-400 (PEG-400), or tetrabutylammonium hydroxide (TABOH) catalyst in the presence of KOH is larger than that using KOH only. However, the conversion of allyl bromide for the alkylation of MBI by tetrabutylammonium bromide (TBAB), tetrabutylammonium hydrogensulfate (TBAHS) and tetrabutylammonium iodide (TBAI) catalyst in the presence of KOH is less than that using KOH only. On the basis of the experimental evidence, a rational reaction mechanism is proposed and a kinetic model is derived, from which a pseudo-steady-state hypothesis (PSSH) is applied to the reaction system. The kinetic behaviors and the characteristics of the reaction are sufficiently described by the pseudo-first-order rate law. This investigation covers the effects on the conversion of the reactant and the apparent rate constants (k(app)) due to the reaction conditions, including the agitation speed, amount of TBAOH (or QOH) catalyst, amount of KOH, quaternary ammonium salts, volume of water, volume of dichloromethane, amount of 2-mercaptobenzimidazole, amount of allyl bromide, organic solvents, and temperature.