Many thiols have strong odours resembling that of garlic. The odours of thiols are often strong and repulsive, particularly for those of low molecular weight. Skunk spray is composed mainly of low molecular weight thiol compounds. These compounds are detectable by the human nose at concentrations of only 10 parts per billion.
However, not all thiols have unpleasant odours. For example, grapefruit mercaptan, a monoterpenoid thiol, is responsible for the characteristic scent of grapefruit. This effect is present only at low concentrations. The pure mercaptan has an unpleasant odour.
Natural gas distributors began adding thiols, originally ethanethiol, to natural gas, which is naturally odourless, after the deadly 1937 New London School explosion in New London, Texas. Most gas odourants utilized currently contain mixtures of mercaptans and sulfides, with t-butyl mercaptan as the main odour constituent. In situations where thiols are used in commercial industry, such as liquid petroleum gas tankers and bulk handling systems, the use of an oxidizing catalyst is used to destroy the odour. A copper-based oxidation catalyst neutralizes the volatile thiols and transforms them into inert products.