Free energy surfaces for electron transfer reactions between redox centers located at the interface between water and self-assembled monolayers are studied by molecular dynamics computer simulations. The free energy curves are correlated with recent studies on the structure and polarity of the interface. We find that the reorganization free energy is dependent on the degree of roughness and the polarity of the organic monolayer as well as on the depth at which the redox center is imbedded in the monolayer region. (C) 2003 Elsevier B.V. All rights reserved.