Chemical oxidation of (TMP)Mn-III(Cl) (TMP = the tetramesitylporphyrinato dianion) by Fe(ClO4)(3) leads to the porphyrin-oxidized product (TMP.+)Mn-III(ClO4)(2). Magnetic measurements and EPR spectroscopy show that the total spin of the complex changes from S = 5/2 at high temperature to S = 3/2 at low temperature. Ligand exchange of the perchlorato ligands in (TMP.+)Mn-III(ClO4)(2) by chloride anions is accompanied by a change of the oxidation site from porphyrin to metal, resulting in (TMP)Mn-IV(Cl)(2). This high-valent-metal complex can effect chlorine atom transfer to olefins, as well as to dimedone and chlorodimedone, natural substrates of chloroperoxidases.