The redox behavior of the self-assembled monolayer of 4-ethoxy-4＇-((N-(10 "-mercaptodecyl)amino)carbonyl)azobenzene (CH3CH2O-Ph-N=N-Ph-C(O)NH(CH2)(10)-SH, abbreviated as C(2)AzoC(10)SH below) on a gold electrode surface has been investigated by electrochemical in situ Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). The bands observed in the potential dependent IRRAS spectra corresponded well to the reduction/oxidation process of the azobenzene group in the monolayer. No bands were found in the in-situ FT-IRRAS spectra for the C-H stretch of the methylene group and the N-H stretch as well as the C=O stretch of the amide moiety, suggesting that no orientation change occurs in the polymethylene chain of the monolayer during the redox reaction of the azobenzene group. The band intensity observed in the in-situ IRRAS spectra depends on the reaction time, especially in the reduction process of the azobenzene monolayer. Our results provided direct evidence for a slower reduction of azobenzene to hydrazobenzene than the reverse process, i.e., oxidation of hydrazobenzene, thus augmenting the importance of understanding the relationship between monolayer structure and electron transfer.