Tetrabutylammonium hydroxide (Bu4NOH, or QOH) is employed as the catalyst to increase the conversion and to enhance the rate of reaction of o-phenylene diamine and carbon disulfide both in the homogeneous as well as the two-phase solution. The alkalinity of the catalyst is an important factor in affecting the reaction. The mechanism of the reaction, which takes place only in the organic phase, was verified by two steps. The reaction of CS2 with QOH first takes place to form the intermediate QCS(2)OH. Then this intermediate reacts with o-phenyIene diamine to produce 2-mercaptobenzimidazole (MBI). A kinetic model based on the reaction is considered for the homogeneous-phase and the two-phase reaction. A pseudo-steady first order rate law is employed to simplify the kinetic model. A high reaction rate and a large conversion is observed when the reaction is carried out in a homogeneous-phase solution using protic or aprotic solvent. Nevertheless, the MBI product precipitates from the two-phase reaction solution. Simple separation processes are sufficient to remove the product from the mixed solution.