Self-assembled layers of 3-mercaptopropionic acid deposited on Au(1 1 1) from solution have been investigated with synchrotron based, high resolution X-ray photoemission spectroscopy. Results obtained from pristine, X-ray irradiated and heated samples are reported. Pristine samples are characterized by an intense, dominant S 2p state at a binding energy of 162 eV (2p 3/2 level) assigned to thiolates. A less intense doublet around 163.3 eV is tentatively attributed to physisorbed molecules forming a partially occupied upper layer. Irradiation and heating cause the development of a third S 2p doublet around 161 eV which, by comparison with the literature, is assigned to atomic sulfur generated by S-C bond scission. The decrease of the C 1s and O 1s signals with annealing and irradiation, together with the increase of the S 2p state assigned to atomic sulfur, confirms the breaking of the C-S bond.