Mercury and its compounds are found throughout natural history museums: in geological/paleontological collections as integral parts of the specimens or their matrices; in biological and anthropological collections as residues of pest control treatments; and in biological collections as a residue of some preparation methods. Mercurials may volatilize toxic vapor and pose a threat to museum staff. Some convert to a black sulphide which can alter the integrity of specimens from aesthetic, scientific, and preservation perspectives. First, the authors briefly review the chemistry of mercury and its compounds and the reactions which they can undergo with many of the materials found in biological collections. We then present data from XRD, XRF, x-radiography, and SEM-EDS analyses used to indicate strongly the presence of mercury sulphide and other chemicals on the specimens, and postulate some reaction pathways. However, due to the numerous chemical treatments and environmental factors to which specimens have been exposed, we cannot yet define a mechanism for conversion of mercury salts to sulphides.