the invention provides a catalyst composition suitable for hydrogenation of polymers containing ethylenic unsaturation , which comprise : a titanium compound of the formula , ## str1 ## wherein a 1 and a 2 are the same or different and each represents a substituted or unsubstituted cyclopentadienyl or indenyl group , wherein x 1 and x 2 are the same or different and each represents hydrogen , halogen , a lower alkyl or lower alkoxy , optionally substituted phenyl or phenoxy , or aralkyl having from 7 to 10 carbon atoms , or phenylalkoxy having from 7 to 10 carbon atoms , carboxyl , carbonyl , a -- ch 2 p 2 , -- ch 2 si 3 , or -- p 2 group . an alkalimetal hydride , added as such or prepared in situ in the polymer solution from the alkalimetal terminated living polymer and / or from additionally added alkalimetal alkyl and hydrogen , the molar ratio of the alkalimetal : titanium in the polymer solution during hydrogenation being at least 2 : 1 ; a borium compound b wherein the symbols r 1 , r 2 and r 3 may be the same or different and each may represent hydrogen , halogen , lower alkyl or lower alkoxy , or phenyl optionally substituted by up to five substituents selected from halogen and lower alkyl , or benzyl having an optionally substituted phenyl ring as specified hereinbefore or wherein two of the symbols r may form together a monocyclic or bicyclic system which on its own may carry one or more substituents . a process for hydrogenation of polymers containing ehylenic unsaturation using these catalyst compositions is also described .