The passivating properties of self-assembled monolayers derived from the chemisorption of the rigid rod arenethiols 1a-b on Au(lll) have been probed by electrochemical techniques in an aqueous electrolyte. These studies suggest that the pure monolayers of 1a-b possess a high density of unfilled defect sites, potentially as a result of the solvent required for their preparation. However, a heterogeneous "patching" process, which involves both addition of hexadecanethiol at defect sites and exchange with the original monolayer, provides mixed monolayers that have superior passivating properties and which X-ray photoelectron spectroscopy and grazing-angle IR reveal to be still predominantly composed of 1a-b. The ability to reproducibly obtain these highly blocking mixed monolayers in which 1a-b is the primary component now provides a means by which to investigate, at the next level of study, electron transfer/tunneling through these novel interfacial barriers.