Voltammetric studies-of c-type cytochromes at clay-modified pyrolytic graphite electrodes have been undertaken as providing a good model for investigating interactions of proteins with soils. Different clays have been investigated according to their global charge, composition and hydrophobicity. The electrochemical behavior of either mitochondrial or bacterial cytochromes at these modified electrodes has been used to define a general rule for the nature of interactions between cytochromes and clays. The influence of various parameters such as pi values of proteins, pH, ionic strength, composition of the clay coating, on the electrochemical response is reported. It is demonstrated that while electrostatic interactions are responsible for the electrochemical promotion of cytochrome c, hydrophobic factors play a key role in the electron transfer process on multiheme cytochromes at clay-modified electrodes. Finally, the consequences of these interactions on the metal reductase activity developed by bacterial polyheme c-type cytochromes have been discussed.