Which compound causes the characteristic rotten-egg smell?

The compound that causes the characteristic rotten-egg smell is hydrogen sulfide.

Hydrogen sulfide is a chemical compound containing two hydrogen atoms bonded to one sulfur atom, giving it the formula H₂S. It is a colorless gas with a distinctive odor at very low concentrations. The smell is often associated with decaying organic matter because microorganisms can produce it when breaking down sulfur-containing materials without oxygen.

Hydrogen sulfide occurs naturally in places such as swamps, sewers, volcanic gases, and some natural-gas deposits. It can also form in industrial processes, including petroleum refining and wastewater treatment. Although its odor is recognizable, smell is not a reliable safety warning because high exposure can quickly impair the sense of smell.

The gas is toxic and flammable, so suspected exposure requires leaving the area and following professional safety procedures. It should not be confused with sulfur dioxide, which has a sharp, choking odor and contains two oxygen atoms instead of hydrogen.