The kinetic properties of the conversion of CO2 to formic acid with formate dehydrogenase (FDH) using some dithionite-reduced 2,2'-bipyridinium salts (1,1'-ethylene-2,2'-bipyridinium dibromide diquat and 1,1'-dimethyl-2,2'-bipyridinium dichloride) as an artificial coenzyme were studied. By using the reduced form of diquat, the effective CO2 reduction to formic acid with FDH is accomplished. The catalytic efficiency, the k(cat)/K-m value of the reduced form of diquat was ca. 126 times larger than that of NADH.