The potential dependent structural change and irreversible anodic decomposition of the self-assembled monolayers of 3-mercaptopropanenitrile (HSC2CN) and of 8-mercaptooctanenitrile (HSC7CN) on gold electrode were investigated by two modes of electrochemically modulated infrared spectroscopy, namely, the subtractively normalized interfacial Fourier transform infrared reflection-absorption spectroscopy (SNIFTIRS) and the difference spectra with only one potential alteration. In addition to the bands due to water molecules around 3100-3500 cm-1 and 1600-1700 cm-1, bands were observed around 2930, 2850, 2342, and 2250 cm-1 at both the HSC7CN and HSC2CN modified gold electrodes. Except for the band at 2342 cm-1, all the bands were observed only by using p-polarized light, suggesting these bands are due to adsorbed species. The 2930 and 2850 cm-1 bands are of the asymmetric and symmetric C-H stretching modes of methylene groups, respectively. The intensities of these two bands decreased a little as potential became more positive. These results suggest that the alkyl chain stands closer to surface normal when potential became more positive. The peak position of the 2250 cm-1 band shifted slightly to lower frequency than that of the same band in neat liquid of corresponding mercaptoalkanenitrile and was not affected by potential so much. On the basis of these results the 2250 cm-1 band was assigned as the CN stretching mode of the terminal CN group, which has no direct interaction with gold. The fact that the absorbance decreased as potential became more positive suggests that the orientation of the terminal nitrile group became closer to parallel to the surface. The 2342 cm-1 band is due to CO2 in solution generated by an irreversible oxidative decomposition of monolayer as this band was observed even by using s-polarized light and grew in the potential region where anodic current flowed. The irreversible anodic decomposition of the monolayer in positive potential region was confirmed by XPS measurements. Effects of alkyl chain length on both the potential dependence of peak position and intensity and the nature of anodic oxidation were also discussed.