Chemical modification of unsaturated polysiloxanes with pendant or terminal vinyl groups and polycarbosilanes with pendant Si-vinyl groups has been achieved by a novel ruthenium catalyzed reaction. Dihydridocarbonyltris(triphenylphosphine)-ruthenium catalyzes the addition of the ortho C-H bond of 2’-methylacetophenone across the C-C double bond of pendant or terminal vinyl groups of polymers such as copoly(dimethylsiloxane/vinylmethylsiloxane), polydimethylsiloxane which is terminated with vinyldimethylsiloxy groups and copoly(vinylmethylsilylene/1,4-phenylene).