Oxidation of the NADH analogue, N-benzyl-1,-4-dihydronicotinamide (BNAH), by the 1e(-) acceptor, [Os(dmb)(3)](3+), and 2e(-)/2H(+) acceptor, benzoquinone (Q), has been investigated in aqueous solutions over extended pH and buffer concentration ranges by application of a double-mixing stopped-flow technique in order to explore the redox pathways available to this important redox cofactor. Our results indicate that oxidation by quinone is dominated by hydride transfer, and a pathway appears with added acids involving concerted hydride-proton transfer (HPT) in which synchronous transfer of hydride to one O-atom at Q and proton transfer to the second occurs driven by the formation of the stable H(2)Q product. Oxidation by [Os(dmb)(3)](3+) occurs by outer-sphere electron transfer including a pathway involving ion-pair preassociation of HPO42- with the complex that may also involve a concerted proton transfer.