The phase-transfer catalyzed reaction of o-phenylene diamine and carbon disulfide to synthesize 2-mercaptobenzimidazole (MBI) in a gas/alkaline solution of KOH two-phase medium was carried out in a semi-continuous reactor. Tributylamine of competitive price was employed as the catalyst to promote the reaction rate. The reaction is also enhanced by the addition of KOH to the reaction mixture. Gas-phase carbon disulfide of higher temperature is continuously introduced to the reactor for the reaction. The main advantage of the present system is that the reaction is carried out at relatively high temperature in the semi-continuous reactor. Carbon disulfide first dissolves in the organic phase and reacts with tertiary amine in the organic-phase solution to produce an active intermediate (R3N-CS2). Then, this active intermediate further reacts with o-phenylene diamine to produce the desired product MBI. Based on the experimental observation, a reaction mechanism is proposed. A kinetic model which considered the two-step reaction was developed. A pseudo first order rate law is sufficient to describe the reaction. The effect of the reaction conditions, such as: the amount of tributylamine (TBA), agitation speed, initial concentration of carbon disulfide, now rate of gas phase, amount of o-phenylene diamine and temperature, on the conversion and the apparent rate constant are investigated in detail.