Density functional theory has been applied to identify possible reaction intermediates for the catalytic C=C hydrogenation of cinnamaldehyde, which occurs in acidic aqueous solutions in the presence of water-soluble ruthenium phosphine complexes. On the basis of ONIOM calculations, two different active species and, hence, two pathways were proposed. The C=C bond hydrogenation takes place through the insertion of the terminal carbon atom into the Ru-H bond and subsequent protonation of the other carbon by hydroxonium ions present in the solution. We find that water is directly involved in several steps of the reaction, either as a protonating/deprotonating agent or as a coordinating ligand. Selectivity against C=O hydrogenation is due to the much higher barrier of either C insertion or O insertion into the Ru-H bond as compared to that of the C insertion in the case of the C=C functionality. © 2006 American Chemical Society.