A new ruthenium-based catalytic system for the tail-to-tail dimerization of acrylonitrile has been developed. The addition of carboxylic acids effectively increased the selectivity toward linear dimers, while suppressing the formation of propionitrile, an undesired by-product. Thus, under optimized conditions, the selectivity for linear dimers reached 85.8%. The carboxylic acids are considered to be involved in the protonolysis of the ruthenium-carbon bond of precursors to the product dimers. In accordance with this hypothesis, a linear relationship was observed between the logarithm of the relative rate of dimerization and the pK(a) value of substituted benzoic acids. It has also been found that the new catalytic system promotes decomposition of 2-cyanoethyl carboxylate, a main Michael addition by-product in the dimerization, to acrylonitrile and the corresponding carboxylic acid under the dimerization conditions. This leads to an increase in the selectivity for linear dimers. A plausible mechanism for the dimerization is proposed. (c) 2006 Elsevier B.V. All rights reserved.