AbstractThe electron impact ionization mass spectra of 4,5‐bis(alkylthio)‐1,3‐dithiole‐2‐thiones and their 1,2‐dithiole‐3‐thione isomers were studied by accurate mass measurements and linked scans. The relative abundance of ions formed following the extrusion of S2, CS or CS2allows an unambiguous isomer differentiation. Isomerization of molecular ions was studied by means of metastable ion analysis and collision‐induced dissociation. The order of reactivity was analogous to that observed in isomerization under photoch