ESR, optical and NMR measurements have been carried out to study the reactions occurring between Fe(III)TPP(−OCH3)2 (complex A) and t- or n-butylhydroperoxide in DMSO. The addition of t- or n-BHPO to complex A, in aerobic conditions, caused a marked shift of the absorption maxima, and resulted in the disappearance of the ESR signal due to the formation of diamagnetic species. The observed optical spectrum of the new iron complex (complex B) is explained as the Fe(II)TPP(−OCH3)2 species. In addition, the results of NMR measurements justified that complex B takes the ferrous low-spin state (S=0). The presence of an intermediate species (complex C), in the reduction processes of complex A to B, was demonstrated by means of simultaneous ESR and optical measurements, combined with the rapid mixing and freezing treatment. From comparison of the optical and ESR parameters of complex C and related iron complexes, the complex was ascribed to be six-coordinate Fe(III)TPP(−OCH3)(−OO-butyl), in which the deprotonated peroxide anion binds at the axial position. After standing the reaction mixture at 298 K for about 1 min, the ESR signal due to complex C disappeared, and an identical optical spectrum to complex B was observed. This means that the Fe(III)TPP-peroxide complex can be reduced to complex B, in which it is in the ferrous low-spin state. Based on these observation results, a possible reaction mechanism for autoreduction of complex C will be discussed.