Quantum mechanical analysis reveals that carbonyl reduction of aldehydes and ketones by the imine-based reductant cis-[Os-III(tpy)(CI)(NH = CHCH3)(NSAr)] (2), which is accessible by reduction of the analogous nitrile, occurs by hydride-proton transfer (HPT) involving both the imine and sulfilimido ligands. In carbonyl reduction, water or alcohol is necessary to significantly lower the barrier for proton shuttling between ligands. The -N(H)SAr group activates the carbonyl group through hydrogen bonding while the -NC(H)CH3 ligand delivers the hydride.