The electrocatalytic reduction of L-cystine to L-cysteine and the reduction of the oxidized form of glutathione to the reduced form of glutathione by the Fe'TSPP species was investigated in aqueous solution. The electrocatalytic oxidations of L-cysteine to L-cystine by FeTSPP, Mn(4-TMPyP), iron(I1) tris (3,4,7,8-tetramethyl-l,lO-phenanthroline) and iron(H) tris( l,lO-phenanthroline) were investigated in aqueous buffer solutions at room temperature. The water-soluble iron tetrakis(4-sulfonatophenyl)porphine (FeTSPP) catalyzed the rapid reduction of L-cystine to L-cysteine after using an electrochemical method to reduce Feii'TSPP and the iron (III) p-0x0 dimer (TSPP)Felll-O-Fel'l(TSPP) to the Fe'TSPP species. Iron(H) tris( l,lO-phenanthroline) and iron tris(3,4,7,8-tetramethyl-l,lO-phenanthroline) may perform the electrocatalytic oxidation of L-cysteine to L-cystine. Fe"'TSPP was reduced to Fe"TSPP by L-cysteine or the reduced form of glutathione. L-cysteine or the reduced form of glutathione were oxidized in a basic solution. If [Fe1i1(TSPP)13-is oxidized to [Fe1u(TSPP'+)]2-by electrochemical method, it can oxidize L-cysteine to L-cystine electrocatalytically or oxidize the reduced form of glutathione in strong acidic solutions. If Mn"i(4-TMPyP) is also reduced by L-cysteine to Mni'(4-TMPyP), L-cysteine oxidizes to L-cystine. The electrocatalytic oxidation of L-cysteine to L-cystine by Mn(4-TMPyP) is performed by two different pathways. Mn"(4-TMPyP) is oxidized to either Mnu1(4-TMPyP) or Mn'"(4-TMPyP) by the electrochemical method, then Mn'"(4-TMPyP) or Mn"'(4-TMPyP) oxidizes L-cysteine to L-cystine. An electrocatalytic oxidation of L-cysteine similar to the oxidation by Mn'"(4-TMPyP) or Mn"'(4-TMPyP) can also be performed by the reduced form of glutathione.