Hydrogenation is an important method of chemical modification, which improves the physical, chemical, and thermal properties of diene elastomers. Natural rubber latex (NRL) can be quantitatively hydrogenated to provide a strictly alternating ethylene-propylene copolymer using a homogeneous osmium catalyst OsHCl(CO)(O-2)(PCy3)(2). A detailed kinetic investigation was carried out by monitoring the amount of hydrogen consumption during the reaction using a gas-uptake apparatus. The kinetic results of NRL hydrogenation indicated that this system had a secondorder dependence of the hydrogenation rate on hydrogen pressure and then decreased toward a zero-order dependence for hydrogen pressures above 13.8 bar. The hydroge-nation was also observed to be first-order with respect to catalyst concentration and inverse first-order on rubber concentration due to impurities present in the rubber latex. Additions of a controlled amount of acid demonstrated a beneficial effect on the hydrogenation rate of NRL. The temperature dependence of the hydrogenation rate was investigated and an apparent activation energy (over the range of 120-160 degrees C) was calculated as 57.6 kJ/mol. Mechanistic aspects of this catalytic process are discussed on the basis of kinetic results. (c) 2006 Wiley Periodicals, Inc.