We demonstrate a very fast heterogeneous redox reaction of substrate-free cytochrome P450(cam) on a glassy carbon electrode modified with sodium montmorillonite, The linear relationship of the peak current in the cyclic voltammogram with the scan rate indicates a reversible one-electron transfer surface process. The electron transfer rate is in the range from 5 to 152 s(-1) with scan rates from 0.4 to 12 V/s, respectively, These values are comparable to rates reported for the natural electron transfer from putidaredoxin to P450(cam). The formal potential of adsorbed P450(cam) is -139 mV (vs NHE) and therefore positively shifted by 164 mV compared to the potential of substrate-free P450(cam) in solution. UV-VIS and FTIR spectra do not indicate an influence of the clay colloidal particles on the heme and the secondary structure of P450(cam) in solution. However, P450(cam) adsorbed on the surface of the clay-modified electrode may undergo partial dehydration resulting in the shift of the formal potential.