The rhodium bis-olefin complex [C5Me5Rh(C2H3SiMe3)(2)], 1, has been shown to be a catalyst for the selective addition of olefins to the ortho position of aromatic ketones. The addition of vinyltrimethylsilane to benzophenone was studied by NMR spectroscopy, which indicated that 1 was the catalyst resting state for this process. This reaction was applied to a series of olefins (allyltrimethylsilane, 1-pentene, norbornene, 2,2'-dimethyl-3-butene, cyclopentene, and vinyl ethyl ether) and aromatic ketones (benzophenone, 4,4'-dimethoxybenzophenone, 3,3'-bis(trifluoromethyl)benzophenone, dibenzosuberone, acetophenone, p-chloroacetophenone and p-(trifluoromethyl)acetophenone). The dependence of the turnover frequency on substrate concentration was investigated. In the presence of excess ketone the formation of a benzophenone complex, 5, [(C5Me5Rh)(2)-eta(4)-eta(4)-C6H5C(O)Ph] was observed after consumption of olefin. Active catalyst is regenerated upon addition of olefin to 5. On the basis of kinetic experiments and labeling studies using acetophenone-d(8), a mechanism for this catalysis is proposed. In the reaction of acetophenone with vinyltrimethylsilane and 1, the formation of ethylene and the trimethylsilyl enolate of acetophenone was observed together with the alkyl-aryl coupling product. This side reaction is specific for vinyltrimethylsilane.