The magnitude of the total rate constant of an electrochemical reaction at small overpotentials is shown to depend on the exponentials of the distance between the electron donor and acceptor, the reorganization energy, and the formal potential of the reaction. When a population distribution of one or more of these three parameters exists, the effect on the measured standard rate constant can be illuminated by using small-amplitude a.c. electrochemical impedance spectroscopy. Assuming a Gaussian distribution of the logarithm of the standard rate constants, the results from the system of cytochrome c molecules adsorbed on thiol-modified gold electrodes yield a standard deviation of approximately 0.5 for log k degrees/k(avg)degrees. The peak broadening of reversible voltammograms is related to a comparable distribution of formal potentials.