Dense monolayers of [Os(bpy)(2)(p3p)(2)](2+), where bpy is 2,2’-bipyridyl and p3p is 4,4’ trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and the area occupied per molecule suggests that only one of the p3p ligands binds to the electrode surface, the other being available for protonation. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron transfer rate constant k for the Os-2+/3+ redox reaction. For electrolyte concentrations above 0.1 M, heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s(-1)). Tafel plots of the dependence of In k on the overpotential eta show curvature, and larger cathodic than anodic rate constants are observed for a given absolute value of eta. This response is consistent with electron transfer occurring via a through-space tunneling mechanism. Plots of k vs pH are sigmoidal, and the standard heterogeneous rate constant k(o) decreases from (6.1 +/- 0.2) x 10(4) to (1.6 +/- 0.1) x 10(4) s(-1) as the pH of the contacting solution is decreased from 5.05 to 1.07. When in contact with pH 5.05 electrolyte, the electrochemical enthalpy Delta H-double dagger is 37.5 +/- 2.1 kJ mol(-1), which decreases to 24.6 +/- 1.5 kJ mol(-1) at a pH of 1.07. The reaction entropy Delta S-rc(o) is independent of the pH over this range, maintaining a value of 82 +/- 7 J mol(-1) K-1. In contrast to the behavior expected from the decrease of k with decreasing pH, the free energy of activation Delta G(double dagger) decreases with decreasing pH. The electronic transmission coefficient kappa(el), describing the probability of electron transfer once the nuclear transition state has been reached, is considerably less than unity for all pH’s investigated. kappa(el) decreases with decreasing solution pH, suggesting an increasingly weaker electronic interaction between the metallic states of the electrode and the orbitals of the redox center as the monolayer becomes protonated. These results suggest that monolayer protonation modulates the heterogeneous electron transfer rate by changing the through-space electron transfer distance. This may be caused either by a change in the tilt angle between the adsorbate and the electrode or by the methylene spacer units within the bridging ligand becoming extended, when the monolayer is protonated.