The faradaic impedance of the surface redox system benzo(c)cinnoline-dihydrobenzo(c)-cinnoline is studied experimentally in aqueous medium between pH 5 and 13. The variations of the impedance components are in good accord with the theoretical predictions. A V-shaped curve is found for log k s =f(pH) ( k s =rate constant of the surface electrochemical reaction). It is estimated that the determination of rate constant values up to 2×10 4 s −1 on a mercury electrode is possible by this method.