Reactive sulfane sulfur species such as persulfides and H2S2 are important redox regulators and linked to H2S signaling, but their study is hindered by a lack of suitable donors to produce them. Here, the authors report 2H-thiopyran-2-thione sulfine (TTS), a compound which can specifically convert H2S to HSOH, and then to H2S2 in the presence of excess H2S.