The electron impact ionization mass spectra of 4,5-bis(alkylthio)-l,3dithiole-2-thiones and their 1,'tdithiole-3thione isomers were studied by accurate mass measurements and linked scans. The relative abundance of ions formed following the extrusion of S, , CS or CS, allows 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 photochemical conditions.