For the catalyst system of ruthenium and carboxylic acid, which is useful for the efficient tail-to-tail dimerization of acrylonitrile, the TON increases as the ruthenium catalyst concentration is decreased. Furthermore, the addition of aromatic solvents of equal volume to that of acrylonitrile improves the catalyst activity. Especially, the use of diphenyl ether leads to a 1.7 time improvement of the TON.