Various macrocyclic cobalt complexes were chemically bonded to a glassy carbon (GC) electrode, on which CO, electroreduction was carried out to give CO in aqueous phosphate buffer solution of pH 6.3. The macrocyclic cobalt complexes were naphthalocyanato cobalt(II), phthalocyanato cobalt(II), dibromo(11-hydroxyimino-4,10-dipropyl-5,9-diazatrideca-4,9-dien-one oximato) cobalt(III) (denoted as CoDO), two kinds of hydrophobic vitamin B(12)s (heptamethyl cobyrinate perchlorate and heptapropyl cobyrinate perchlorate), 5,10,15,20-tetraphenylporphyrinato cobalt(II), and 5,10,15,20-tetrakis (4-methoxyphenyl)porphyrinato cobalt(II). Their redox potentials of Co(I)/Co(II) are in the above order from positive to negative potentials in a 0.05 M TBAP DMSO solution, being between -0.23 and -1.0 V (SCE). These complexes were chemically bonded to GC through -CONH-pyridine which locates perpendicularly to a planar or semi-planar complex structure, where the N of the pyridine forms a coordinate bond with the Co atom of the above complexes as a fifth ligand. The catalytic activity for hydrogen evolution in aqueous solution was observed to be high on Co naphthalocyanine and Co phthalocyanine modified GC electrodes; the latter gave H-2 evolution at the most positive potentials among the Co complexes employed. When the lower potential limit in cyclic voltammetry became less than the hydrogen evolution potential, in the reverse positive-going sweep, an anodic hump current was observed at -0.35 similar to -0.78 V, which is assigned to a cobalt hydride oxidation process; the hydride is suggested to form when hydrogen evolution takes place, and the hump disappeared after the introduction of CO2 into the solution. It was observed that the Co complex chemically bonded on GC can give CO from CO2 only at relatively low overvoltages, except for CoDO which was not able to reduce CO2; phthalocyanato cobalt(II) gave CO at E = -1.0 V (0.26 V as overvoltage) at 20% current efficiency. The highest CO current efficiency was observed in the case of tetraphenyl-porphyrinato cobalt(II) chemically modified GC.