Electron tunneling (ET) through alkanethiol bilayers trapped between two small mercury drops (Hg-Hg tunneling junction, geometric area: 8 x 10(-4) cm(2)) was investigated. Self-assembled monolayers were formed on Hg drops using one- or two-component solutions of n-alkanethiols (ranging from nonanethiol to hexadecanethiol) in hexadecane. Mercury drops covered by monolayers were brought into contact using micromanipulators. Current-voltage cyclic curves were used to measure the capacitance and tunneling current for the alkanethiol bilayer. The experimental current-voltage curves were compared with the following theoretical models: classical Simmons theory, Simmons theory modified by including effective electron mass (Lindsay's model), and a model for off-resonance tunneling through a molecular bridge (Ratner's model). The classical Simmons theory does not fit our tunneling data while Lindsay's and Ratner's models agree reasonably well with the tunneling characteristics of Hg-Hg junctions. The electrical properties of two-component bilayers, containing a mixture of hexadecanethiol and nonanethiol deposited on each Hg drop, were studied as a function of a monolayer composition. The thickness of the two-component monolayer on each Hg drop depends linearly on the mole-fraction of nonanethiol. ET through a two-component system is less efficient than ET through single-component bilayers. This result is rationalized in terms of diminished electronic coupling through van der Waals contacts. (C) 2002 Elsevier Science B.V. All rights reserved.