The cobalt protoporphyrin-IX reconstituted myoglobin (CD-Mb) was prepared and its electrochemical and spectroelectrochemical properties were studied in comparison with native myoglobin. Unlike native myoglobin, a slow electron transfer reaction for Co(III)-Mb/Co(II)-Mb was detected by cyclic voltammogram and spectroelectrochemical methods. The chemical reduction rate of Co(III)-Mb with dithionite was also slow compared with that of native myoglobin. Direct electron transfer between Go-Mb and an In2O3 electrode was observed, with spectroscopic verification of different redox states of Go-Mb using an optically transparent thin layer electrode (OTTLE) cell. The reversible electrochemical redox reaction of Co(III)-Mb/Co(II)-Mb was observed using azure A, 3-amino-7-dimethylaminophenazathionium chloride, as an electron transfer mediator for the first time. The Nernst plot was obtained for Co-Mb with E-O' of - 100 mV versus Ag \ AgCl (sat. KCI) and n = 1 at 25 degrees C.