The electrochemical transformation of the self-assembled monolayer of 4-aminothiophenol (4-ATP) has been studied in neutral solution by means of electrochemical and in situ FTIR measurements, and the results are compared with those in acidic solution. The electrochemical transformation in neutral solution results in the formation of a surface-confined aniline dimer, 4'-mercapto-N-phenylquinone diimine as in the case of acidic solution. The redox peak corresponding to the aniline dimer shifts by -56 mV while increasing the solution pH by 1 unit, suggesting that protons and electrons take part in the redox reaction in the ratio of 1 to 1. The surface coverage of the aniline dimer obtained in the neutral solution is relatively low ((1.3 +/- 0.3) x 10(-10) mol cm(-2)) when compared to that in 0.1 M HClO4 solution ((2.4 +/- 0.3) x 10(-10) mol cm(-2)). Very similar in situ FTIR spectral features were obtained in both acidic and neutral solutions, indicating that the reaction mechanism follows the same head-to-tail coupling in neutral solution as in the case of acidic solution. The electrochemical and FTIR spectroscopic studies of the model compound, N-phenyl-1,4-phenylene diamine, support the formation of aniline dimer on the electrode surface during the electrochemical transformation of 4-ATP in neutral and acidic solutions. The electroproduced diimine undergoes hydrolysis to yield the monoimine while holding the electrode at high positive potential.