A study of inhibitors in the dimerization of acrylonitrile using the novel catalytic system of [RUCl2(DMSO)(4)/CH3CH2COONa/DMSO/o-benzoylbenzoic acid] was carried out by examining the effect of reaction products on the catalytic activity. 1,4-Dicyanobuta-1,3-diene (3), one of the linear dimer products of acrylonitrile, inhibited the Ru-catalyzed reaction. Methylthiomethyl o-benzoylbenzoate (8), a by-product which was produced from carboxylic acid and DMSO under the reaction conditions, also acted as an inhibitor. (C) 2008 Elsevier B.V. All rights reserved.