Polyhalogenated benzenes have been analyzed by standard electron capture chemical ionization mass spectrometry (ECCI/MS), and electron monochromator mass spectrometer (EM/MS) systems. The EM/MS experiments distinguished electronegative compounds (especially isomers) by transient negative ions (TNIs) of the same mass but differing energies and/or the formation of distinct TNIs of different masses for each electronegative compound. The ECCI/MS results of polybrominated benzenes show that "bromine addition" and hydrogen abstraction reactions occur to form ions with one to three bromines added to the starting material. Computations have given insight into the mechanisms that may be involved in the EM/MS and ECCI/MS experiments. Calculations have also provided qualitative support for the processes that may be occurring in the ECCI/MS experiments, and in non-photochemical and photochemical reactions of polyhalogenated benzenes. Photochemical experiments reveal that a minor, but notable pathway that involves an "iso-polychlorobenzene intermediate", is taking part in the photo-dehalogenation of chlorobenzene and hexachlorobenzene. / Graduation date: 2000