The reaction between H2O2 and a pyridine-coordinated ferric porphyrin encapsulated by a cyclodextrin dimer yielded a hydroperoxoferric porphyrin intermediate, PFeIII-OOH, which rapidly decomposed to oxoferryl porphyrin (PFeIV=O). Upon reaction with H2O2, PFeIV=O reverted to PFeIII-OOH, which was converted to carbon monoxide-coordinated ferrous porphyrin under a CO atmosphere. PFeIV=O in the presence of excess H2O2 behaves as PFeIII-OOH.