The impediment of heterogeneous electron transfer (ET) reactions of several electroactive species by alkanedithiol self-assembled monolayers (SAMs) and those covered with Au nanoparticles was investigated. It was found that, when densely packed 1,6-hexanedithiol and 1,9-nonanedithiol SAMs were formed with and without adsorbed gold nanoparticles, Fe(CN)(6)(3-) reduction became extremely sluggish. The attachment of Au nanoparticles does not appear to improve the electronic communication between the solution species and the underlying Au electrode surface. On the other hand, Ru(NH3)(6)(3+) and ferrocenecarboxylic acid both exhibited quasi-reversible redox waves at these surfaces. The variation of impediment were attributed to the different electron transfer mechanisms for these redox species. The adsorption of Au nanoparticles onto alkanedithiol SAMs was estimated by flow injection quartz crystal microbalance (FI-QCM), showing a rapid adsorption process. Amounts of Au nanoparticles did not vary between the two different alkanedithiols. Finally, we quantified the adsorption of An nanoparticles at the QCM crystal modified with a 1,9-nonanedithiol SAM in air. A surface coverage of 2.56% was obtained. (C) 2004 Elsevier Ltd. All rights reserved.