Asymmetric I/V curves with respect to the polarity of the voltage bias were observed in the Hg-Au junctions containing bilayers of alkanethiols of different chain length. A larger current resulted when a negative bias was applied to the metal carrying a longer chain alkanethiol monolayer. This behavior is simulated using a single molecule junction model, within the frameworks of the extended Huckel (EH) model and the nonequilibrium Green's function formalism at the Hartree Fock level (NEGF-HF). Qualitative agreement with the experimental results with respect to the magnitude and sign of this asymmetry is obtained. On the basis of the NEGF-HF calculation, the origin of the effect is suggested to be the asymmetric behavior of the character of the junction highest occupied molecular orbital (HOMO) at opposite biases. This change of character leads to different effective barriers for electron transfer for opposite signs of the voltage drop across the junction. (C) 2003 Elsevier B.V. All rights reserved.