Bis-phosphine Ru(II)-arene complexes are highly active and chemoselective catalysts for the hydrogenation of aldehydes in the presence of olefinic bonds. Mechanistic studies, including comparisons with a structural analog, reactions of an isolated hydride complex, base poisoning expts., and a computational anal., help rationalize the preferential hydrogenation of C=O bonds, which probably proceeds via an ionic outer-sphere mechanism.