Polymer hydrogenation process

An improved process for the selective hydrogenation of the carbon-carbon double bonds in a copolymer of a conjugated diene and one or more copolymerizable monomers which comprises hydrogenating said copolymer at a temperature of from about 35.degree. to about 100.degree. C. and at a pressure of hydrogen of from about 0.05 to about 7 MPa the improvement being that said hydrogenation is carried out in the presence of from about 2.5 to about 40 percent by weight of a catalyst which is a monovalent rhodium hydride complex of the general formula RhHL.sub.x in which x is 3 or 4 and L is a ligand compound wherein when x is 4, L is a phosphorus compound selected from 5-phenyl-5H-dibenzophosphole and compounds of the formula PR.sub.1 R.sub.2 R.sub.3 and when x is 3, L is an arsenic or antimony compound of the formula MR.sub.1 R.sub.2 R.sub.3 wherein M is arsenic or antimony and wherein R.sub.1, R.sub.2 and R.sub.3 may be the same or different and are selected from CH.sub.3, C.sub.2 H.sub.5, C.sub.6-10 aryl groups and C.sub.7-12 aralkyl groups, the hydrogenation being carried out in an organic solvent in which said copolymer and said catalyst are soluble and in the absence of any additional amounts of said ligand compound, the concentration of said copolymer in the solution being from about 0.05 to about 20 percent by weight based on the total weight of said solution.