A high-frequency ac admittance technique capable of precisely measuring rate constants greater than 1 cm s(-1) was used to collect kinetic data for both the one-electron reduction and one-electron oxidation of tetrakis-(dimethylamino)-p-benzoquinone. This study was conducted at a gold ultramicroelectrode in seven polar aprotic solvents, with varying viscosity, in the presence of 0.1 M tetraethylammonium perchlorate. In addition, the temperature dependence of the standard rate constants was determined in three of these solvents. On the basis of an analysis of the solvent effects, it is shown that the kinetic data show a variation in the degree of reaction adiabaticity and the relative sizes of both the inner- and outer-sphere components of the Gibbs energy of activation. Solvent effects on diffusion coefficients and standard potentials were also investigated. The standard reduction potential is a linear function of the solvent acceptor number, a parameter reflecting molecular solvating properties, whereas the corresponding standard potential for oxidation is a linear function of solvent polarizability, a parameter that reflects nonspecific interactions between the reactant and the solvent. The significance of these results is discussed within the context of the current models for simple heterogeneous electron transfer reactions.