The phase-transfer catalyzed reaction of o-phenylene diamine and carbon disulfide to synthesize 2-mercaptobenzimidazole (MBI) in a gas phase/liquid-phase solution two-phase medium was carried out in a semi-continuous reactor. Tetrabutylammonium hydroxide (TBAOH or QOH), which is an active compound, was used as the catalyst to promote the reaction rate. The liquid-phase solution contains the mixture of ethanol and water in an appropriate volume ratio. The gas-phase CS2/CH2Cl2 mixture of a higher temperature was continuously introduced to the reactor for reaction. The main advantage of the present system is that the reaction is carried out at a relatively higher temperature in the semi-continuous reactor. The gas-phase CS2/CH2Cl2 mixture first dissolves in the EtOH/H2O liquid-phase solution and reacts with QOH in the liquid-phase solution to produce an active intermediate (HOCS(2)(-)Q(+)). 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 considers the two-step reactions was developed, and a pseudo first order rate law is sufficient to describe the reaction. The effects of the reaction conditions, such as the amount of QOH, the agitation speed, the initial concentration of carbon disulfide, the flow rate of gas phase, the amount of o-phenylene diamine, the volume ratio of EtOH/H2O and the temperature, on the conversion of o-phenylene diamine and the apparent rate constant (k(app)) are investigated in detail.