The regioselective hydrogenation of cinnamaldehyde in basic aqueous media catalyzed by [RuH2(mtppms)x [x -3,4; mtppms = meta-sulfonatophenyl-diphenylphosphine] is analyzed by means of theoretical calculations. The water solvent is modeled by the inclusion of a (H 2O)3 cluster in addition to a continuum model. Two Ru complexes are evaluated as active species for the reduction process: the major identified species [RuH2P4] and the related phosphine dissociated complex [RuH2P4]. Several reaction mechanisms are computationally evaluated, and their analysis suggests that the reaction takes place in several steps. The first hydrogenation process takes place by means of a hydrogen transfer from the metal catalyst, whereas the second hydrogenation process is performed by a water solvent molecule. This mechanism can account for the selectivity of the C=O versus C=C double bond reduction experimentally observed under basic reaction conditions and can be directly related to the mechanism found in acidic media, where the opposite regioselectivity (C=C vs C=O) is observed. © 2006 American Chemical Society.