The electrochemical behaviour of thin films of nitrogenated amorphous carbon a-CNx is similar to that of boron-doped diamond, with a wide potential window in aqueous media. They are elaborated by cathodic sputtering of a graphite target in an Ar-N2 active plasma for varying nitrogen contents, determined by XPS (0.06 ≤ x ≤ 0.39). Their electrochemical reactivity is sensitive to the surface state. The present study reports on the influence of electrochemical pre treatment on the electronic transfer rate of a fast redox system ferri-ferrocyanide, by focusing on the direction of the potential excursion. On the other hand, the role of both the pH and the potential on the interfacial capacitance in the presence of Na2SO4 without redox species is documented. The results show up the sensitivity of the film surface to the electrochemical conditions.