Clay-modified electrodes (CMEs) were prepared with synthetic smectite clays containing only one metal (magnesium, nickel or iron). A voltammetric wave was observed in the electrode modified with the iron clay in the absence of any adsorbed electroactive species. No such wave has ever been reported for natural smectites. It is attributed to direct electron transfer between redox-active structural Fe(II) sites in the clay lattice and the conductive substrate. This voltammetric wave was much larger when some nickel was incorporated in the lattice of the synthetic iron clay. The current obtained for a clay containing 14% nickel was more than six times larger than for the pure iron clay. However, no peaks were found for the pure nickel clay or for nickel clays containing small amounts of iron. The redox-active sites in the synthetic iron clay were shown to relay electrons between [Ru(NH3)(6)](2+) and [Fe(bpy)(3)](3+) cations coadsorbed in the CME, forming the beginning of an electron transport chain in the clay films. No evidence of mediated electron transfer was found for the iron-free magnesium clay. Variations of the Fe(II)/Fe(III) ratio in films of natural smectites during cyclic voltammetry was also followed by UV-visible spectroscopy.