The Shvo catalyst is one of the most prominent examples of a hydrogen-transfer catalyst successfully applied in a broad scope of hydrogen-transfer processes. The reaction takes place by transferring a hydride (bonded to the metal center) and a proton (bonded to a ligand) to a double bond. The reaction mechanism for the hydrogen-transfer process, however, is a matter of controversy. Experimental studies by means of primary deuterium isotope effects on the hydrogenation of ketones via the active reducing form of the Shvo catalyst by the Casey and Bäckvall groups concluded that carbonyl hydrogénation is concerted. Nevertheless, it is not clear whether the reaction goes through an outer-sphere mechanism (without substrate coordination) or through an inner-sphere mechanism (with substrate coordination). In the present work several inner- and outer-sphere mechanisms are explored by means of theoretical methods. Energy reaction profiles clearly support the outer-sphere mechanism. Theoretically combined KIEs also suggests the concerted outer-sphere mechanism for this reaction. Some interesting features on the behavior of the Ru complex were found during the mechanistic investigations. © 2007 American Chemical Society.