Manganese(III) tetrakis(N-methyl-4-pyridyl)porphine (MnTMPyP) is water-soluble and exhibits a reversible Mn-III/II redox couple in acidic solutions. However, the Mn-IV/III couple cannot be seen by cyclic voltammetry in aqueous solutions of pH < 8. Spectroelectrochemical methods have shown that ((H2OMnTMPyP)-Mn-)-T-III undergoes a one-electron oxidation with slow heterogeneous electron transfer rate to form O = (MnTMPyP)-T-IV, which then oxidizes cyclopent-2-ene-1-acetic acid (1) catalytically to give cyclopent-2-ene-4-one-1-acetic acid (2). Further electrochemical oxidation of O = (MnTMPyP)-T-IV causes the deactivation of the catalyst, presumably due to porphine ring degradation. A sterically hindered porphyrin, manganese(III) tetrakis(sulfonatomesityl)porphine (MnTSMP), is first oxidized at the porphine ring and is stable as a radical cation (H2O)(MnTSMP+.)-T-III in pH < 2 solutions. In pH 8.5 buffer solutions, (MnTSMP)-T-II is oxidized totally by two electrons at E-appl = +1.05V. The electrogenerated O = (MnTSMP+.)-T-IV reacts rapidly with 1 to give 2 and cyclopent-2,3-diol-1-acetic acid (3). The reaction mechanisms are proposed.