Nitrogen-doped hydrogenated amorphous carbon, a-C:H:N-x films, were synthesized by microwave electron cyclotron resonance plasma-enhanced chemical vapor deposition, using N-2/C2H2 as precursors. Such films, with thickness in the range of a few tens of nanometers, very smooth, and pinhole free, appear as electrode materials with reactivity and effective window potential similar to those of boron-doped polycrystalline diamond or nitrogen-doped amorphous diamond. These films, grown on Si or Ti substrates, were investigated with regard to their electrochemical reactivity toward outer sphere redox reactions, such as ferri/ferrocyanide, by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). It has been established that the electrical conductivity and the electrochemical reactivity vary in opposite directions, the optimum found for the former being around a nitrogen atomic fraction of 0.3, and for the latter around 0.05. EIS showed that for a fast redox reaction, the behavior, though looking similar to a simple electron transfer on a metallic substrate with a partial diffusion control, has another contribution, which could be explained by the presence of a layer at the interface with the solution, in which electron transport would occur by hopping between many localized states. Ti substrate improves the electrical contact with the carbon film with respect to the Si substrate, which decreases the apparent potential difference between the anodic and cathodic redox peaks by about 150 mV. The minimum value of 10(24) cm s(-1) found for the charge-transfer rate constant of Fe-II/III lies in the range so far obtained for other modified carbon materials, such as boron-doped polycrystalline diamond or nitrogen-doped amorphous diamond thin films.