Alkanethiol self-assembled monolayers (SAMs) on a mercury surface are used to build a junction consisting of two opposing mercury surfaces with interposed SAMs: Hg-SAM/SAM-Hg. The liquid mercury surface provides a support for the SAM that is smooth, compliant, free of defects, and without the incommensurate lattice properties that characterize solid metal surfaces. The thickness of the dielectric ( ∼ 30–90 Å) in this junction can be easily changed by using alkanethiols with different lengths. From capacitance measurements, a dielectric constant of 2.7±0.3 is calculated for the SAMs. The conductivity of SAMs on the Hg surface is σ = 6±2×10−15 Ω−1 cm−1, a value close to that of bulk polyethylene. The junction sustains an electric field of 6 MV/cm. © 1998 American Institute of Physics.