Methacrylonitrile was polymerized under cathodic polarization onto nickel electrodes. Compact polymer films strongly bound to the metal were obtained when using strictly controlled organic electrolytic solutions. The polymerization process was found to be highly dependent on the monomer concentration in the electrolytic solution. For a low monomer concentration (less than 30% of the volume) the reduction mechanism was dominated by a diffusion process occurring at a Very negative potential (-2.8 V versus AgAg+). No adsorption pre-peak appeared and no visible film was formed on the electrode. For a high monomer concentration and especially for a pure methacrylonitrile-supporting salt solution the electrochemical process was characterized by a well-defined adsorption-reduction prepeak located at a more positive potential (-2.3 V). The polymer film formed on the electrode was found to be insoluble in the monomer but highly soluble in solvents specially in acetonitrile. On the contrary, when using a moderate monomer concentration (between 30% and 90% of the volume) the major part of the electropolymer formed under the adsorption peak was dissolved in the electrolytic solution; however, a thin (600 Angstrom) insoluble compact and covering film remained strongly bound to the electrode. In order to explain the complex behaviour of the polymerization process, different polymer arrangements on the electrode depending on the monomer concentration are proposed and discussed.