The electrochemistry of a system containing a surface-confined electroactive species is characterized by electrochemical impedance spectroscopy, and the results are displayed on a Cole-Cole plot. The rate of adsorption of electroactive horse-heart cytochrome c to a carboxythiol-modified gold electrode is experimentally monitored by continuously measuring the change in the magnitude of the pseudocapacitance, which is directly proportional to the amount of adsorbed protein. The time dependence of the adsorption of cytochrome c to the thiol-modified gold electrode shows a significant period of diffusion-controlled behavior. Limitations on the frequency of measurement are outlined in relationship to the magnitude of the standard rate constant of the electron-transfer reaction.