Surface-enhanced Raman spectroscopy (SERS) has been used to characterize monolayers of p-substituted benzenethiols (XBTs) and p-substituted benzenemethanethiols ((XBMTs) (substituent = X = F, Cl, Dr) on gold electrodes. Detailed vibrational assignments have been made for the Raman and SER spectra of all six compounds. All of these molecules exist on the surface as thiolates, with the aromatic ring tilted relative to the surface normal. Monolayers of the XBTs and XBMTs remain intact on the surface throughout the potential range between the oxidation of the gold surface at similar to +800 mV vs SCE and the reduction of water at ca. -1000 mV at neutral pH. Monolayers of ClBT and BrBT can be partially reduced electrochemically to form mixed monolayers of the halogenated BT and benzenethiol itself. The reductive elimination of the halide occurs at potentials more positive than are required for reduction of the same molecules in solution. FBT, FBMT, and ClBMT cannot be reduced at the surface, and the BrBMT monolayer is only slightly reduced. The electrochemical reactivities of the XBT and XBMT monolayers are explained in terms of facilitated electron transfer from the metal to the adsorbed thiolate, the properties of the leaving group (halogen), and the electronic consequences of having a methylene spacer group between the sulfur and the aromatic ring. This work shows the feasibility of modifying aromatic self-assembled monolayers in situ to form mixed monolayers. It also provides a framework for designing and fabricating monolayers with prescribed stabilities and electroactivities.