Various selenium- and sulfur-containing volatiles have been detected and characterized as products of chalcogen exchange reactions. The formation of all possible selenium- and sulfur-containing trichalcogenide isomers (-SeSS-, -SSeS-, -SeSeS-, -SeSSe- and -SeSeSe-) was observed at room temperature in the solutions containing diselenide and trisulfide. Because of the low activation barrier of the selenium exchange reaction, these species are expected also to form in biological systems in the presence of diselenides and higher sulfides. Methyl and ethyl derivatives of these species were characterized using gas chromatography coupled to time-of-flight mass spectrometry with electron impact, chemical, and field ionization. Rearrangements of triselenides lead also to the formation of isomeric branched-structure triselenides, selanadiselenides. Because of the very similar structural properties, the behavior of these novel species was studied under various ionization modes (EI+, CI+, EI-, and CI-).