Grapefruit mercaptan is a natural organic compound found in grapefruit. It is a monoterpenoid that contains a thiol functional group. Structurally, a hydroxy group of terpineol is replaced by the thiol in grapefruit mercaptan, so it is also called thioterpineol. Volatile thiols typically have very strong, often unpleasant odors that can be detected by humans in very low concentrations. Grapefruit mercaptan has a very potent, but not unpleasant, odor, and it is the chemical constituent primarily responsible for the aroma of grapefruit. This characteristic aroma is a property of only the R enantiomer. The aroma of the racemate is described as "sulfurous, aromatic, grapefruit".