Mixed self-assembled monolayers of ferrocene, triarylamine or phenothiazine terminated alkanethiols were prepared on gold-ball electrodes. Also, a ferrocene terminated biphenylthiol and anthracenethiol were investigated. The heterogeneous electron transfer rate constant was measured in all cases by impedance spectroscopy between 1 MHz and 0.1 Hz. For equal spacer lengths, we obtained significantly higher rate constants for the triarylamine species compared to the ferrocene compounds, which is attributed to the much lower reorganisation energy of triarylamines. (c) 2006 Elsevier B.V. All rights reserved.