The interactions of mercury on cleaved and polished galena PbS(0 0 1) were studied at room temperature. The surface of the samples was characterised by X-ray photoelectron spectroscopy (XPS), RHEED and X-ray absorption spectroscopy (EXAFS) before and after interaction with a pressure of 2.5 x 10(-3) mbar of mercury vapours. Sorbed mercury was oxidised to Hg(II), as revealed by the chemical shift observed on the Hg 4f(7/2) levels. Since the adsorption of mercury is completely inhibited when the oxidation products of galena cover the surface, the formation of Hg-S bonding is most likely to occur. The amount of adsorbed mercury at saturation was estimated to be about one monolayer, i.e. one atom of mercury per surface atom of sulphur. A (I x 1) structure was observed by RHEED for the longest exposures. The analysis of the EXAFS oscillations at the L-III edge of mercury allowed an estimate of the Hg-S bond length of 2.62 Angstrom and a co-ordination number of 3. A model is proposed for the structure of the chemisorbed layer of mercury.