Cyclic voltammetry was used to investigate the kinetics of the electron transfer between uranium(VI) and polyheme c-type cytochromes of bacterial origin. Reduced cytochromes were oxidized by U(VI), either in homogeneous solution, or when cytochromes were entrapped by a dialysis membrane or immobilized within an ion-exchanger film. By using membrane electrode technology, several factors such as uranium complexation, pH, origin of the cytochrome and medium were examined. Results suggest that uranium(VI) reductase activity is strongly dependent upon experimental conditions. In particular, a narrow pH window and well-controlled complexation conditions are necessary to observe high electrocatalytic reduction of uranium(VI) by c-type cytochromes. It has been demonstrated that U(VI) electroreduction originates in the presence of hemin-containing groups in the partner species.