A new homogeneous catalyst precursor, OsHCl(CO)(L)(PCy3)(2) (1, L vacant; 2, L = O-2). has been identified for the selective hydrogenation of olefin within acrylonitrile-butadiene copolymers. This report details a study of its catalytic behavior at hydrogenation conditions approaching those used industrially. Carefully acquired kinetic data, collected within a statistical design framework, define the influence of [Os], [RCN], [H-2], [PCy3], and temperature on the catalytic activity. Reversible coordination of nitrile to complex 1 not only reduces the hydrogenation rate but creates an unprecedented sensitivity of the process to pressure. Unique to this system is an apparent second-order dependence of the hydrogenation rate on [H-2], which diminishes toward zero order as pressures exceed 60 bar. In contrast, hydrogenation of substrates that lack nitrile is virtually indifferent to [H-2] at all pressures. These kinetic measurements, coupled with recently acquired knowledge of the structures and reactivities of complexes 1 and 2, provide the basis of a plausible reaction mechanism.