The S-alkylation of 2-mercaptobenzimidazole (MBI) by -bromo-m-xylene (RBr) under phase transfer catalytic reaction condition was successfully carried out in an aqueous solution of KOH/organic solvent two-phase medium. No product was obtained from N-alkylation during or after the reaction period by using a limited quantity of -bromo-m-xylene. The reaction is greatly enhanced by adding a small amount of tetrabutylammonium bromide (TBAB) and/or potassium hydroxide. Based on the experimental evidence, a rational reaction mechanism is proposed. 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. The effects of the reaction conditions, including agitation speed, amount of TBAB catalyst, amount of KOH, quaternary ammonium salts, volume of water, volume of dichloromethane, amount of 2-mercaptobenzimidazole, amount of -bromo-m-xylene, inorganic salts, organic solvents, and temperature on the conversion of the reactant and the apparent rate constants (kapp) were investigated in detailed. Rational explanations are provided for the observed phenomena from experimental results.