The heterogeneous electron transfer kinetics for spontaneously adsorbed monolayers of [Os(bpy)(2)Cl(pNp)]-PF6, where bpy is 2,2’-bipyridyl and pNp is 1,2-bis(4-pyridyl)ethane or 4,4’-trimethylenedipyridine, have been explored using short time scale potential step chronoamperometry. For the Os2+/(3+) redox reaction, heterogeneous electron transfer is a rapid first-order process characterized by a single unimolecular rate constant (k/s(-1)). Temperature-resolved measurements of k have typically been made over the temperature range -5 to +40 degrees C and have been used to determine the ideal electrochemical enthalpy, Delta (H) over bar(I)(double dagger). The ability of the Butler-Volmer equation to accurately describe the potential dependence of the enthalpy and entropy of activation has been investigated. The electrical component of Delta (H) over bar(I)* is considerably less sensitive to potential than is predicted by the Butler-Volmer formulation. The influence of solvent on the kinetics and thermodynamics of electron transfer has been explored in acetonitrile, acetone, dimethylformamide, dichloroethane, tetrahydrofuran, and chloroform. For the p2p monolayers, the standard rate constants k degrees range from 7.4 x 10(3) s(-1) in chloroform to 1.1 x 10(5) s(-1) in acetonitrile, while for the p3p monolayers the k degrees values are 1.6 x 10(3) and 1.8 x 10(4) s(-1), respectively. For both these monolayers, a linear correlation is observed between In k degrees in ln tau(l), where tau(l) is the longitudinal relaxation rate of the solvent. The slope of the plot is negative and near unity, suggesting that electron transfer is strongly influenced by solvent reorganization dynamics. The ideal electrochemical activation enthalpy has been determined as a function of the solvent for the p3p monolayer.