The mass spectra of several di- and tetra-chlorobiphenyls suggested randomization of chlorine over both phenyl rings in the molecular ion prior to fragmentation. The exceptions, 2,2′- and 2,6-dichloro-, 2,2′,4,4′-, 2,2′,5,5′-, 2,3,5,6-, and 2,2′,6,6′-tetrachloro-biphenyls, all contain two or more chlorine atoms ortho to the Ph–Ph bond and their metastable ion intensities which result from the M→M– Cl2 reaction are markedly different from those of the other isomers. The primary ion spectra of most isomeric compounds are similar, with successive losses of Cl· from the molecular ion. This process is often accompanied by expulsion of HCl from the lower chlorine homologues (2)–(5) but only the reactions which expel Cl· are accompanied by metastable ions. The ion kinetic energy spectra of the di- and tetra-chloroisomers confirm the suggested fragmentation pathways.