In organic chemistry, hemithioacetals (or thiohemiacetals) are organosulfur compounds with the general formula . They are the sulfur analogues of the acetals, , with an oxygen atom replaced by sulfur (as implied by the thio- prefix). Because they consist of four differing substituents on a single carbon, hemithioacetals are chiral. A related family of compounds are the dithiohemiacetals, with the formula . Although they can be important intermediates, hemithioacetals are usually not isolated, since they exist in equilibrium with thiols () and aldehydes ().