Chromium(III) meso-tetrakis(3-sulfonatomesityl)porphyrin ((CrTSMP)-T-III) in aqueous solution exhibits two pK(a)s at 8.5 and 11.6, respectively. The redox reactions depend on the pH in the solution. In acidic solutions, the first oxidation at +0.875 V (versus Ag/AgCl) is a one-electron step corresponding to the porphyrin ring cation radical formation. The first two oxidations are separated in pH < 4.0 but merge together in the pH 6.0-8.0 region. The two-electron oxidation step, of which the potential is pH dependent, involves the metal and the ring, and splits in two in pH > 9.0 solution. A possible internal electron transfer between the metal center and the porphyrin ring occurs in the pH 9.4-12.5 region. O=(CrTSMP)-T-IV is not reactive towards alkene oxidation while O=(CrTSMP+.)-T-IV reacts with alkene to give enone at room temperature. The spectroelectrochemical results have been confirmed by chemical methods. The electrochemical reduction occurs at the porphyrin ring first and the second reduction is metal centered.