The acid-base reaction of aminoethanethiol (2AT) and 6-amino-1-hexanethiol (6AT) self-assembled monolayers (SAMs) On a gold electrode was monitored by Fourier-transform surface enhanced Raman spectroscopy (FT-SERS). A band attributed to counter anions, such as ClO4- or NO3- for NH3+, was clearly observed during protonation of the amino group of 2AT or 6AT. The band intensity decreased with increasing solution pH. The surface pK(a)'s of the 2AT and 6AT SAMs on a gold electrode at 0 V vs Ag/AgCl (saturated KCl) were 5.0 +/- 0.2 and 3.8 +/- 0.3, as evaluated from the intensity-pH curve.