The reduction potential for the {sup {sm bullet}}CO{sub 2}{sup {minus}} radical anion has been determined by equilibration of formate with sulfhydryl radicals of {beta}-mercaptoethanol, penicillamine, and lipoamide in aqueous solutions at pH 3-6. The reaction {sup {sm bullet}}CO{sub 2}{sup {minus}} + e{sup {minus}} + H{sup +} = HCO{sub 2}{sup {minus}} yields the value E{degree}{sub 9} = 1.49 V with an uncertainty of {plus minus}0.06 V. On the basis of this value and the known free energies of CO{sub 2}(aq) and HCO{sub 2}{sup {minus}}(aq), E{degree}{sub 19} for CO{sub 2} + e{sup {minus}} = {sup {sm bullet}}CO{sub 2}{sup {minus}} was found to be -1.85 V.