Electrocatalytic carbon dioxide reduction was studied in aqueous media on glassy carbon electrode coated with polymeric Co-tetra-3-aminophenylporphyrin. The polymeric complex-modified electrode catalyses the carbon dioxide reduction to carbon monoxide and formic acid. The monomeric system does not catalyze the reduction probably because it is not stable. The polymer, instead, presents higher activity at lower potentials and loses its activity only after more than four hours of electrolysis at constant potential. The redox couple responsible for the catalysis is Co(II)/Co(I). Kinetic electro-reduction parameters for the polymer-modified electrode were obtained and a probable mechanism operating in aqueous media containing sodium perchlorate as electrolyte is proposed