We report herein the fabrication of a molecular junction in which a thin (8−15 Å) layer of oriented organic molecules is positioned between two electronic conductors. The molecular layer becomes a component in an electronic circuit and exhibits properties that depend strongly on molecular structure. Bonding between the carbon substrate and the molecular layer is covalent and conjugated, and thus differs fundamentally from that of the widely studied self-assembled monolayers of alkane thiols on metal surfaces. The chemisorbed molecular layer is densely packed and stable and does not contain the tunneling barrier imposed by a sulfur atom. The current/voltage behavior of methyl-phenyl, n-butyl phenyl, tert-butyl phenyl, and stilbene monolayers between pyrolytic carbon and mercury indicates a negligible pinhole density and shows weak dependence on temperature. The action of a nitroazobenzene molecular junction as a bistable switch is demonstrated, and switching behavior persisted for many on−off cycles and over a period of at least 14 h. Carbon-based molecular junctions represent a new paradigm for molecular electronics, which shows promising electronic behavior and is amenable to low cost, benchtop processing.