The electron impact mass spectra of fifteen 4-substituted PBN's and PBN itself have been obtained using the probe method. Strong molecular ion peaks were observed in most cases. Fragmentation follows two major pathways depending on the polarity of the substitutent: thetert-butyl bond breaks to produce either thetert-butyl cation or isobutylene. The latter route is analogous to the well known McLafferty rearrangement. Fragmentation to produce thetert-butyl radical is a minor route. Electron-donating polar substituents appear to enhance fragmentation to isobutylene.