The kinetics of NAD(+) reduction was investigated on a ruthenium modified glassy carbon electrode (RuGC) in wide polarization rate, concentration, temperature, and pH ranges using the electrochemical techniques of cyclic and differential pulse voltammetry, chronoamperometry and chronopotentiometry. It was shown that the modification of GC by a sub-monolayer of Ru can provide an electrode surface capable of reducing NAD(+) directly to NADH, avoiding the formation of a dimer. The reaction is highly irreversible, and occurs at high negative overpotentials, where the reaction rate is controlled by the surface diffusion of electroactive species. The reaction is pH independent. It was postulated that Ru sites have a bifunctional role, serving as both proton-providing sites, and as a possible physical barrier for dimerization of NAD-free radicals. A set of kinetic and thermodynamic parameters was calculated and verified independently using various experimental techniques: the standard heterogeneous electron-transfer rate constant, the apparent transfer coefficient, the apparent diffusion coefficient, the reaction order, and the standard Gibbs energy of activation. (C) 2004 Elsevier B.V. All rights reserved.