We have investigated the formation behavior and electrochemical properties of mixed self-assembled monolayers (SAMs) composed of 11-ferrocenyl- 1-undecanethiol (11-FUT) and hexanethiol molecules formed using an exchange method. Two redox peaks were recognized in the cyclic voltammograms, indicating that 11-FUT molecules on gold substrates have two different states. The invariability of the peak potential observed at about 360 mV versus Ag/AgCl (3M NaCl) evidenced that the local packing density in some parts of 11-FUT molecules is constant, irrespective of the time for the exchange process; that is, the immersion time in the 11-FUT ethanol solution. This is accounted for by the formation of the domains of 11-FUT molecules. STM images showed the formation of nano-domains when the hexanethiol molecules were replaced with 11-FUT molecules. These results suggest that the redox peak splitting observed in cyclic voltammograms is due to the difference in the local density of the 11-FUT molecules on the substrates. (c) 2007 Elsevier Ltd. All rights reserved.