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 be...