We report the electrochemistry of surface-confined monolayers of 4-aminothiophenol (4-ATP) and mixed monolayers of 4-ATP and thiophenol (TP) on Au surfaces. Cyclic voltammograms of the 4-ATP monolayer in acidic aqueous perchlorate solutions are characterized by an irreversible oxidative wave at 0.730 V vs Ag/AgCl on the first scan and, upon scan reversal, by a persistent, reversible, surface-confined wave centered at approximately 0.500 V and a transient peak at about 0.300 V. We propose an ECE mechanism to account for this electrochemistry. 4-ATP is first oxidized to the cation radical, followed by chemical coupling to form an adsorbed dimer. The dimer is subsequently hydrolyzed in the presence of H2O to yield an adsorbed quinone species that is reversible electroactive. Grazing angle FTIR spectroscopy was used to identify the product. The transient peak is due to the coupling of desorbed molecules and is consistent with the formation of a phenazine species. We then show that mixed monolayers of 4-ATP and TP can be used to study the coverage dependence of surface-confined reactions. The chemical composition of the mixed systems was determined using two independent Auger electron spectroscopic techniques and grazing angle FTIR spectroscopy. Using 20 min assembly times, we find that the surface concentration of 4-ATP is directly proportional to its mole fraction in solution. Interestingly, TP does not participate in the 4-ATP electrochemistry and functions only to dilute the surface concentration of 4-ATP. We find that the efficiency of the conversion of 4-ATP to product is somewhat higher at low mole fractions of 4-ATP.