Abstract: The catalytic activity of several water-soluble metallo-porphyrin complexes, M(Ⅱ)[p-N(CH₃)₃]TPPI (M = Co, Ni, Cu, Zn; [p-N(CH₃)₃]TPPI = tetrakis (4-trimethylammoniophenyl) porphyrin iodide), for the electroreduction of CO₂ has been studied by means of controlled-potential electrolysis in aqueous solutions. It was found that Co( Ⅱ )[p-N(CH₃)₃]TPPIis active but other metalloporphyrins show no activity. In the presence of Co( Ⅱ )[p-N(CH₃)₃]TPPl the main product of CO₂ reduction was CO, but small amounts of HCOOHwere obtained. The effects of electrolytes, catalyst concentrations, electrode materials and potentials on the reduction of CO₂ were investigated.