A new type of catalyst system using ruthenium and carboxylic acid is useful for the tail-to-tail dimerization of acrylonitrile, proceeding without the formation of undesired by-product propionitrile. Carboxylic acids having pK(a) 3.5-5 are suitable as co-catalysts for the dimerization of acrylonitrile. The relationship between the logarithm of the relative rate in the dimer formation and the pK(a) of m- and p-substituted benzoic acids (Bronsted plot) was linear (R-2 = 0.946) with a slope of -0.199. The role of the carboxylic acids can be considered to be effective protonation in the protonolysis of the carbon-ruthenium bond of an intermediate Ru complex.