Dimerisation process and catalyst

A process for the conversion of acrylonitrile to a product containing more than 50% by weight of adiponitrile, 1,4-dicyano-butenes, or mixtures thereof, which comprises heating acrylonitrile in the liquid phase at a temperature of 50.degree. to 200.degree.C and a pressure of up to 250 atmospheres in the presence of hydrogen or a compound capable of supplying hydrogen under reation conditions and a catalyst consisting essentially of a ruthenium compound co-precipitated with an aluminum oxide or hydroxide in which the proportion of ruthenium to aluminum is in the range 1:40 to 10:1 by weight, said catalyst being obtained by adding a base less strong than alkali metal hydroxide to an aqueous solution to form an alkaline solution containing a water-soluble ruthenium compound and a water-soluble aluminum compound to effect co-precipitation and separating the co-precipitate at a temperature below about 200.degree.C.