The water-soluble cobalt tetrakis(N-methyl-2-pyridyl)porphyrin (Co(2-TMPyP)) catalysed the reductions of tetrathionate ((O3SSSSO3)(2-)), the oxidized form of glutathione (GSSG), and L-cystine (CySSCy) after Co-II(2-TMPyP) was reduced electrochemically to Co-I(2-TMPyP). The electrocatalytic oxidation of thiosulfate (SSO32-), reduced form of glutathione (GSH) and L-cysteine (CySH) are performed through Co-III(2-TMPyP). These reactions occur in a reversible electrocatalytic system using only one catalyst Co(2-TMPyP). An electrocatalytic process is proposed. Co(2-TMPyP) shows two redox couples and can perform both electrocatalytic oxidation and reduction of sulfite in a strong acidic aqueous solution.