The in situ PM-IRAS spectra of adsorbed 11-mercaptoundecanoic acid (MUA) on gold surfaces are reported. PM-IRAS measurements were performed at 70degrees of incidence, through a 1-mum-thick layer of solvent. Spectra taken in various solvents are compared to the one recorded on a dry surface. Observed spectral changes between dry and liquid-phase spectra suggest structural changes in the adsorbed layer, correlated to chemical modifications of the thiol headgroup: deprotonation of the carboxylic groups was observed in basic aqueous or in ethanolic medium, accompanied with a progressive disorder of the chain. This effect was reversible as shown by measurements after re-immersion in a neutral aqueous solution. The use of D2O, as aqueous solvent, permitted the spectral analysis in the nu(C=O) region in neutral, acidic, and basic solutions.