Using [ Ru3( CO),,] (1 ) as the homogeneous precatalyst, transfer hydrogenations of cyclohex-2-en-1 -one, benzylideneaniline, and carbon tetrahalides by donor alcohols, in particular propan-2-01, have been studied. Conversion of cyclohex-2-en-1 -one into cyclohexanol has been found to proceed via the intermediate formation of cyclohexanone. From the temperature dependence of the overall reaction rates, the precatalysts [ Ru3( CO),,] and [ Ru,H,( CO),,] are involved in processes with comparable activation energies. Two catalytically active cluster complexes, [ Ru4( CO),,( C,H,O)] and [Ru,(CO) ,,(C,H,O)], were isolated from the reaction of (1) with cyclohex-2-en-1 -one. Hydrogenation of the tetranuciear cluster led to the formation of [Ru,H,(CO),,] and [Ru,H,-