Dense monolayers of [Os(OMe-bpy)(2)(p3p)Cl](I+), where OMe-bpy is 4,4'-dimethoxy-2,2'-bipyridyl and p3p is 4,4'-trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum, mercury, gold, silver, carbon, and copper microelectrodes. These systems have been used to probe the influence of the electrode density of states on the rate of electron transfer across the electrode/monolayer interface. Monolayers on each material exhibit well-defined voltammetry for the Os2+/3+ redox reaction where the supporting electrolyte is aqueous 1.0 M NaClO4. The high scan rate (> 2000 V s(-1)) voltammetric response has been modeled using a nonadiabatic electron-transfer model. The standard heterogeneous electron-transfer rate constant, k(o), depends on the identity of the electrode material, e.g., k(o) is 6 x 10(4) and 4 x 10(3) s(-1) for platinum and carbon electrodes, respectively. Chronoamperometry, conducted on a microsecond rime scale, has been used to probe the potential dependence of the heterogeneous electron-transfer rate constant. These values range from (4.0 +/- 0.2) x 10(4) to (3.0 +/- 0.3) x 10(3) s(-1) on going from platinum to carbon electrodes. Temperature-resolved chronoamperometry and cyclic voltammetry reveal that the electrochemical activation enthalpy, DeltaH(double dagger), and the reaction entropy, DeltaS(RC)(double dagger), are both independent of the electrode material having values of 11.1 +/- 0.5 kJ mol(-1) and 29.6 +/- 2.4 J mol(-1) K-1, respectively. The effect of electrode material on the preexponential factors is discussed in terms of the electrode density of states. These experimental data indicate that the heterogeneous electron-transfer rate for a nonadiabatic process is not simply proportional to the density of states but is modulated by the electronic coupling efficiency. Moreover, the matrix coupling elements, H-AB, are between 0.1 and 0.5 J mol(-1), which is approximately 4 orders of magnitude smaller than those found from studies of intervalence charge-transfer intensities within comparable dimeric complexes.