The catalyzed reaction of o-phenylene diamine and carbon disulfide to synthesize 2-mercaptobenzimidazole (MBI) by tetrabutylammonium hydroxide (QOH; Q: (C4H9)(4)N) was carried out in a homogeneous solution. A mixture of organic solvent (aprotic or protic) and water of an appropriate volume ratio acts as the homogeneous organic solution. The reaction is greatly enhanced either by tetrabutylammonium hydroxide or by tetrabutylammoniurn bromide in the presence of potassium hydroxide. Effects of the reaction conditions including the concentration of QOH, concentration of KOH, amount Of CS2, organic solvents, volume ratio of organic solvent to water, and temperature on the conversion of o-phenylene diamine are investigated in detail. An explanation is provided for the peculiar phenomena in studying the effects of the concentration of KOH on the conversion of o-phenytene diamine. (c) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.