Mercaptobenzimidazole (MBI) was first synthesized from the reaction of carbon disulfide and o-phenylenediamine catalyzed by tertiary amines in a homogeneous solution. A kinetic model was also proposed, which considered the reaction of carbon disulfide and tertiary amine to form an active intermediate (R(3)N-CS2), and further reacted with o-phenylenediamine to produce the desired product. Thus, an equilibrium reaction of carbon disulfide and tertiary amine was established in the synthesis of MBI. By appropriating choice of the organic solvents, a large conversion of o-phenylenediamine was obtained within a relatively short time interval of reaction. Experimental results indicated that a pseudo-first-order rate law sufficiently expresses the reaction for using carbon disulfide in a large excess amount. Furthermore, a mixture of organic solvents with different polarities was utilized in the reaction to examine the effect of the polarity of solvent on the conversion.