Ru[CH==CH(Ph)]Cl(CO)(PCy3)(2) (where PCy3 is tricyclohexylphosphine) functions as an efficient catalytic system for the hydrogenation of natural rubber. Furthermore, the addition of an acid plays an important role in promoting the hydrogenation process. The quantitative hydrogenation of natural rubber results in a polymer akin to an alternating ethylene-propylene copolymer. Kinetic studies have been carried out via the monitoring of the amount of hydrogen consumed: first-order behavior has been observed with respect to the carbon-carbon double-bond concentration, catalyst concentration, and hydrogen pressure, and an inverse dependence on the added PCy3 concentration has been found. (c) 2005 Wiley Periodicals, Inc.