Complex organosulfur molecules are ubiquitous in interstellar molecular clouds, but their funda- mental formation mechanisms have remained largely elusive. These processes are of critical importance in initiating a series of elementary chemical reactions leading eventually to organosulfur molecules – among them potential precursors to iron-sulfide grains and to astrobio- logically important molecules such as the amino acid cysteine. Here we reveal through laboratory experiments, electronic structure theory, quasi classical trajectory studies, and astrochemical modeling that the organosulfur chemistry can be initiated in star forming regions via the elementary gas phase reaction of methylidyne radicals with hydrogen sulfide leading to thioformaldehyde (H2CS) and its thiohydroxycarbene isomer (HCSH). The facile route to two of the simplest organosulfur molecules via a single collision event affords persuasive evidence for a likely source of organosulfur molecules in star forming regions. These fundamental reaction mechanisms are valuable to facilitate an understanding of the origin and evolution of the molecular universe and, in particular, of sulfur in our Galaxy.