Hydrogenation of polyenes

A process for the hydrogenation of a polyene to a monoene which comprises forming a reaction mixture containing the polyene, hydrogen and a ruthenium-ligand complex hydrogenation catalyst, Ru(PAr.sub.3).sub.2 (CO).sub.2 Cl.sub.2 in which the triarylphosphine ligand P(Ar).sub.3 is selected from the group consisting of P(p-tolyl).sub.3, P(p-F-C.sub.6 H.sub.4).sub.3, P(p-OMe-C.sub.6 H.sub.4).sub.3 or P(C.sub.6 H.sub.5).sub.3, free triarylphosphine and a sterically-hindered tertiary amine promoter of the formula NR.sub.3, where R is alkyl or aryl or a sterically-hindered pyridine having alkyl or aryl groups in the 2 and/or 6 position, and reacting the mixture under hydrogenation conditions, said sterically-hindered tertiary amine or said sterically-hindered pyridine being present in the reaction mixture in an amount in the range of 2 to 10 moles per mole of catalyst.