Iron(II) meso-tetrakis(3-sulfonatomesityl)porphyrin ((FeTSMP)-T-III) contains sterically hindered substituents at the periphery and is water-soluble. The oxidation potential and oxidized forms for FeTSMP vary with pH. In pH < 4.0 solution, a one-electron porphine ring oxidation, which is pH independent, occurs first and is followed by a two-electron oxidation, which is pH dependent. The two-electron oxidation becomes two one-electron reactions as pH increases and one shifts to less positive potential to merge with the porphine ring oxidation. In 4.0 < pH < 6.0, consequently, the first oxidation is a two-electron process and the second oxidation is a one-electron step. Stable high-valent oxo-ferryl porphyrin radical cation was electrochemically generated in aqueous media at room temperature in this pH range. The oxidation pattern then becomes three one-electron steps as pH is higher than 8.0. The high-valent iron porphyrins exhibit different activities towards alkene oxidations in aqueous media. (FeTSMP+.)-T-III does not react with cyclopent-2-ene-1-acetic acid (1). O=(FeTSMP)-T-IV slowly reacts with (1) to form cyclopent-2-ene-4-one-1-acetic acid (2) in the presence of dioxygen while O=(FeTSMP+.)-T-IV reacts rapidly with (1) to form cyclopent-2,3-diol-1-acetic acid (3) in the absence of dioxygen. O=(FeTSMP+.)-T-IV reacts with (FeTSMP)-T-III conproportionally to form O=(FeTSMP)-T-IV, which in turn reacts with (I) to give (2).