A new homogeneous catalyst precursor has been discovered for the hydrogenation of carbon-carbon unsaturation resident within acrylonitrile-butadiene copolymers. The hydrido-phosphine complex OsHCl(CO)(O-2)(PCy3)(2) (1) selectively and quantitatively saturates olefin, leaving the copolymer's nitrile functionality intact. However, the process suffers from an undesirable crosslinking reaction that is not demonstrated by the established rhodium technology. The extent of this crosslinking is dependent on the process conditions and can be minimized by operating with a low catalyst concentration and high H-2 pressure. Kinetic studies have identified a previously unknown unexpected influence of olefin on the polymer crosslinking process. In light of this new information, the prevailing mechanism for this class of reactions has been reconsidered.