Geminal dithiols have the formula RR'C(SH). They are derived from aldehydes and ketones by the action of hydrogen sulfide. Their stability contrasts with the rarity of geminal diols. Examples include methanedithiol, ethane-1,1-dithiol, and cyclohexane-1,1-dithiol. Upon heating, gem-dithiols often release hydrogen sulfide, giving the transient thioketone or thial, which typically convert to oligomers.