A series of mono(bipyridine)carbonylruthenium(II) complexes, [Ru(L1)(CO)2Cl2], [Ru(L1)(CO)2(CH3CN)2][CF3SO3]2, and [Ru(L1)(CO)Cl3][Me4N] (L1 = 2,2’-bipyridine), have been found to act as efficient and selective homogeneous electrocatalysts for the reduction Of CO2 into CO in CH3CN containing 5% water, at a carbon electrode. It has been demonstrated that the electrocatalytic effect is due to the formation of a polymeric film occurring during the electroreduction of the complexes. For [Ru(L1)(CO)2Cl2] and [Ru(L1)(CO)2(CH3CN)2][CF3SO3]2 corresponding films have been identified as the [{Ru0(bpy)(CO)2}n] polymer containing Ru0-Ru0 bonds. These carbon/[{Ru0(bpy)(CO)2}n] modified electrodes appear themselves as efficient molecular cathodes for selective reduction Of CO2 into CO either in CH3CN + 5% water or in pure aqueous electrolyte. The stability of these modified electrodes can be improved by using a preformed polypyrrolic film functionalized with [Ru(L2)(CO)2Cl2] (L2 = 4-(2-pyrrol-1-ylethyl)-4’-methyl-2,2’-bipyridine). In this material the formation of Ru0-Ru0 bonds occurs in the preformed film during its electroreduction.