The direct electrochemistry of N-acetyl-microperoxidase-8 (N-Ac-MP-8) at naked Pt has been investigated in both aqueous and dimethyl sulfoxide solution using cyclic voltammetry. In both aqueous and non-aqueous media, heterogeneous electron transfer has been found to be persistent and at least quasi-reversible. The aqueous redox potential for N-Ac-MP-8 (- 169 +/- 5 mV vs. SHE) is consistent with recent studies establishing that at high ionic strength in aqueous buffered solutions the heme peptide is a low-spin six-coordinate complex in which water occupies the sixth axial ligand site. The redox potential at Pt in dimethyl sulfoxide solutions (< 0.1% H2O), + 103 +/- 5 mV vs. SHE, is in the range of redox potentials typically observed for type II cytochromes c in which the sixth axial ligand binding site is vacant.