Double layer and solvent effects on the kinetics of the one-electron oxidation of Ru(phen)(3)(2+) (where phen = 1,10-phenanthroline) at platinum and gold ultramicroelectrodes were studied by ac-voltammetry. The standard rate constant of Ru(phen)(3)(2+) electrooxidation is independent of both electrode material and concentration of supporting electrolyte, indicating the absence of double-layer effects on the kinetics of electron transfer. The small variation of the formal redox potential of the Ru(phen)(3)(2+/3+) system with change of the supporting electrolyte concentration demonstrates the absence of a significant influence of ion pairing on the reaction rate. Therefore, the electrode process of Ru(phen)(3)(2+) oxidation is found to be ideally suitable for the study of solvent effects on electron transfer kinetics. The solvent dependence of the electron transfer rate constant was interpreted within the context of contemporary theory. The charge transfer process was found to be perfectly adiabatic. Since the crossing rate of the energy barrier is controlled by the dynamics of solvent relaxation, the rate of the electrode reaction primarily depends on the solvent's longitudinal relaxation time or, more approximately, on the viscosity of the solution.