The trans-dihydride complex trans-RuH2(NH2CMe2CMe2NH2)((R)-binap) (1) is an active catalyst for the homogeneous hydrogenation of ketones in benzene under pressure of H-2 gas. The mechanism of the catalysis is proposed to occur through a rapid transfer of a hydride from the ruthenium and a proton from the amine on 1 to the carbonyl of the ketone to give the product alcohol and a hydrido-amido intermediate RuH(NHCMe2CMe2NH2)((R)-binap) (2). The dihydride is then regenerated by the turnover-limiting heterolytic splitting of dihydrogen by complex 2. In this work the kinetic isotope effect (KIE) is measured to be 2.0 (+/- 0.1) for the reduction of acetophenone to 1-phenylethanol catalyzed by 1 using 8.0 atm of H-2 versus D-2 gas. DFT calculations predict a KIE of 2.1 for the described mechanism using the simplified catalyst RuH(NHCH2CH2NH2)(PH3)(2) with H-2 or the catalyst RuD(NDCH2CH2ND2)(PH3)(2) with D-2. This supports the observation that deuterium scrambles into the catalyst when a pressure of D-2 is used. We discuss the significance of these results relative to the KIE of 2 that was reported by Sandoval et al. (J Am. Chem. Soc. 2003 125, 13490) for the hydrogenation/deuteriation of acetophenone catalyzed by trans-RuH(eta(1)-BH4)((S)-tolbinap)((S,S)-dpen) in basic isopropanoVisopropanol-d(8).