The field of molecular electronics is often limited by nonreproducible electrical device characteristics and low yields of working devices. These limits may result from inconsistencies in the quality and structure of the monolayers; of molecules in the devices. In response, the authors have developed an ultrahigh vacuum vapor phase deposition method that reproducibly assembles monolayers of oligo(phenylene ethynylene) molecules (the chemical backbone of many of the molecules used in molecular electronics). To improve the structure and purity of the monolayer, the vapor phase assembly is performed in an ultrahigh vacuum environment using a low temperature organic thermal cell. Because vapor phase assembly does not require the use of solvents, a potential source of contamination is eliminated. The absence of solvents also permits the fabrication of complex device architectures that require photoresist patterning prior to the molecular assembly. Characterization via ellipsometry, x-ray photoelectron spectroscopy, and scanning tunneling microscopy shows that the monolayers are dense, chemisorbed, ordered, and chemically pure. (c) 2007 American Vacuum Society.