Electrochemical and spectroelectrochemical properties of manganese(III) reconstituted myoglobin (Mn(III)-Mb) have been investigated. No redox wave of Mn(III)-Mb was observed at a highly hydrophilic indium oxide electrode on which rapid direct electron transfer of native myoglobin took place, suggesting the electron transfer reaction of Mn(III)-Mb at an indium oxide electrode is very slow. The rate constant of the chemical reduction of Mn(III)-Mb with dithionite was ca. 20 times smaller than that of native Mb. Using an optically transparent thin layer electrode (OTTLE) cell and Oxazine-170 perchlorate, 5,9-bis(diethylamino)-10- methyl-benzo[a]phenoxazonium perchlorate, as an electron transfer mediator, the redox potential for the Mn(III)\Mn(II)-Mb couple was estimated to be -0.32 V versus Ag\AgCl (sat. KCl) at 25 degrees C.