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Cyclic voltabsorptometry is used for the first time to distinguish and characterize electrochemically the active (P450) and inactive (P420) forms of cytochromes P450 immobilized on an electrode during voltammetry experiments. This was achieved by using the heme domain (BMP) of the bacterial cytochrome P450 BM3 from Bacillus megaterium(CYP102A1) immobilized on mesopouros tin-oxide (SnO 2) electrodes. We demonstrate that the formation of either the P450 form or the P420 one can be obtained by modifying the mesoporous electrode surface with polycations with different properties such as polyethylenimmine (PEI) and polydiallyldimethylammonium chloride (PDDA). Potential step spectroelectrochemistry allowed measurement of reduction potentials of the active P450 form. Values of −0.39 ± 0.01 V and −0.58 ± 0.01 V (both versus Ag/AgCl) were calculated for the active P450 form immobilized on the BMP/PDDA−SnO 2and BMP/PEI−SnO 2electrodes, respectively. The cyclic voltabsorptometric experiments showed how, when both the active and inactive forms are present on the PEI film, the inactive P420 species tends to dominate the cyclic voltammetric signal. [ABSTRACT FROM AUTHOR]