The ability of carbon electrodes modified by films of [Ru(L)(CO)(2)](n) (L is a bidentate nitrogen ligand) to act as selective electrocatalysts has been investigated in aqueous solution following the influence of several parameters (electrolyte, initial pH and nature of the ligand L). These organometallic polymers were prepared by electroreduction of trans(Cl)-[Ru(L)(CO)(2)Cl-2] complexes. It has been found that the reaction selectivity is strongly dependent on the 4,4＇-substitutions of the bipyridine (bpy) ligand. For films with a regular bpy ligand or having donor substituents, CO is the main reduction product (eta(CO) = 94%; E-app = -1.20 V at pH 9). With films based on a bpy ligand disubstituted with electron-withdrawing groups like esters the selectivity is drastically changed, since HCOO- can be obtained with a quantitative electrical yield at a very low overvoltage (-0.75 V vs. Ag/AgCl). This difference was attributed to the nature of the intermediates (hydroxycarbonyl or formato complexes) formed during the electrocatalytic process. Utilization of preformed films of polypyrrole functionalized by corresponding monomeric complexes allows the circumvention of problems due to the poor stability of these molecular cathodes toward air oxidation. A similar CO/HCOO- selectivity with regards to the substitution of the ligand by an electron-withdrawing groups is observed.