Interstellar Ice Chemistry Yields Elusive Sulfinothioic Acid (HS(O)SH)

Low‐oxidation‐state sulfur oxyacids are key yet experimentally elusive intermediates in interstellar sulfur chemistry; even the simplest member, sulfinothioic acid (HS(O)SH), has neither been detected in space nor isolated terrestrially. The S–S(O) motif, also present in biological systems, can be reduced to form disulfide bridges (RSSR’), suggesting potential astrobiological relevance. Here, we report the first formation of sulfinothioic acid in astrophysically relevant sulfur dioxide (SO 2 )–water (H 2 O) ices and its identification via isomer‐selective photoionization reflectron time‐of‐flight mass spectrometry combined with isotopic labeling. Irradiation of these ices at 5 K under ultrahigh vacuum with galactic cosmic‐ray proxies yields sulfinothioic acid. These results establish a missing link in interstellar sulfur chemistry and reveal a previously unrecognized class of astrochemically accessible sulfur oxyacids formed via radical‐mediated, non‐equilibrium processes driven by energetic particles in cold interstellar environments.