A deep repository for permanent storage of mercury will be designed and built in Sweden. The preferred chemical state for mercury in such a repository would be the sulphide Hgs (cinnabar), which is a highly insoluble and the dominating natural mercury mineral. The possible formation of HgS from HgO or Hg(O) by reaction with a sulphur source (S, FeS or FeS2) is discussed from thermodynamic considerations, and pe-pH-diagrams are constructed by using the computer code MEDUSA to illustrate under which conditions HgS would dominate. Calculations of the speciation (PHREEQE) under varying conditions (S/Hg-ratios, presence of chloride) are given. Long-term laboratory experiments are performed, where the formation of HgS from the basic components is demonstrated (after mixing under various conditions and storage at room temperature for up to 3 years). The feasibility of HgS-formation with time in a geologic repository under conditions representative of deep granitic bedrock (calcium-bicarbonate buffered to pH 7-8.5) is discussed, as well as effects of alkaline conditions (concrete environment, pH 10.5-12.5). Formation of soluble polysulphides is not expected as long as the S/Hg mole ratio is within 1-1.3 and pH is below 10.5-11. Concrete should be used with caution. Suitable ballast materials could be introduced that would reduce porewater-pH that otherwise would be above 12. (c) 2006 Elsevier B.V. All rights reserved.