The adsorption of D-cysteine on Au(111) surfaces from the solution phase has been investigated by X-ray photoelectron spectroscopy (XPS). To check the reliability of the XPS analysis, we performed measurements on D-cysteine/Au(lll) and alkanethiol/Au(111) interfaces paying particular attention to spectra time evolution during X-ray exposure. In both systems, X-ray irradiation increases the Au-4f intensity, but while in L-cysteine the line shape of the S2p spectra does not change. in 1-octadecanethiol it dots. The analysis of the Au4f and S2p core level regions indicates the occurrence of a dissociative molecular chemisorption on the metal surface for both systems. In the case of L-cysteine, the S2p spectra indicate the presence of a minor sulphur component attributed to the presence of unbound cysteine molecules physisorbed on the first chemisorbed layer.