Electrochemical oxidation of a self-assembled monolayer (SAM) of 4-aminothiophenol on polycrystalline gold electrodes leads to a complex voltammetric behavior characterized by an initial irreversible oxidation at similar to +0.77 V versus SSCE (sodium saturated calomel electrode) and the formation of a pseudostable surface redox couple at +0.53 V. The oxidized form of this couple is hydrolyzed in acidic solutions to another redox pair with the formal redox potential of similar to +0.3 V. We show that the oxidation leads to a radical-radical coupling reaction between two adjacent aminothiophenol molecules, yielding an electrode surface modified with 4＇-mercapto-4-aminodiphenylamine, the thiol derivative of a head-to-tail aniline dimer. The oxidized form of the dimer, quinone diimine, undergoes hydrolysis to the corresponding quinone monoimine and, eventually, to the original surface-bound 4-aminothiophenol and benzoquinone; The mechanism of the monolayer oxidation reaction has been elucidated by a variety of electrochemical and spectroelectrochemical techniques together with electrochemical data obtained with a soluble model compound, 4-(methylthio)aniline. In addition, X-ray photoelectron spectroscopy (XPS) characterization of the 4-aminothiophenol (Au-SPhNH2), the 2-(4＇-mercaptophenylamino)benzoquinone (Au-SPhNH-BQ), and the oxidized 4-aminothiophenol SAMs is reported. The formation of an electrode surface modified with aniline dimers explains the beneficial effect that 4-aminothiophenol SAM exhibits in the electrochemical polymerization of aniline. We suggest that it favors the direct addition of aniline monomers to the oligomer chains on the surface, which results in a more ordered structure compared with the deposition of oligomers from the solution. This result is very important for the preparation of highly ordered polyaniline films for advanced applications in molecular electronics and sensor technology. The results also show that after the initial dimerization step, aniline polymerization can proceed through coupling of the neutral monomer to the oxidized oligomer.