Stereoselective carbon?carbon bond formation is one of the most important types of bond construction in organic chemistry. A mild and acid free catalyst system for the hydrovinylation reaction utilizing a cationic, ruthenium center is described. A catalytic amount of RuHCl(CO)(PCy3)2 (2) activated with AgOTF or AgSbF6 at room temperature was found to be an effective catalyst system for the hydrovinylation of vinylarenes and the intramolecular hydrovinylation (IHV) of 1,6-dienes. Vinylarenes with both electron-donating and electron-withdrawing substituents reacted with ethylene at room temperature to provide the desired 3-arylbutenes in moderate to excellent yield (60-99%) under mild reaction conditions, while the IHV reaction of 1, 6 dienes provided greater than 90% of product conversion. We also developed the first hydrovinylation catalyst containing a chelating, bidentate phosphine ligand that provides the desired product. Our ruthenium-based catalytic system has also proven to give an appealing reactivity profile in favor of the desired arylbutenes without promoting undesirable oligomerization and isomerization.