The reaction mechanism of the Ru(H)(2)(CO)(PPh3)(3)-catalyzed addition of C-H bonds in aromatic esters to olefins; was studied by means of deuterium-labeling experiments and measurement of the C-13 kinetic isotope effect. The deuterium-labeling experiment revealed a rapid equilibrium among the intermediates prior to the reductive elimination, and the C-13 NMR kinetic isotope effect indicated that C-C bond formation is the rate-determining step in this reaction.