Mercaptans or thiols are organic compounds containing the sulfhydryl group –SH attached to a carbon atom. They resemble alcohols, but with the oxygen of the hydroxyl group replaced by sulfur. Mercaptans have an offensive, unpleasant, rubbery, sulfur or skunk-like odor. They are found in improperly prepared or stored wines, crude oils and are added in small concentrations to natural gas and LPG as a safety feature to allow gas leakages to be detected.