Pursuant to the invention a process for the production of hydrogenated nitrile rubber polymers having lower molecular weights and narrower molecular weight distributions than those known in the art in the is provided wherein the process is carried out in the presence of hydrogen and optionally at least one co-(olefin). The present invention further relates to the use of specific metal compounds in a process for the production of a hydrogenated nitrile rubber by simultaneous hydrogenation and metathesis of a nitrile rubber.