The gas chromatographic/mass spectrometric characteristics of 26 congeners of polychlorinated dibenzofurans, previously characterized by specific synthetic routes and by standard spectroscopic techniques, have been evaluated. The electron impact mass spectra are not particularly isomer-specific, though 2,3,7,8-tetrachlorodibenzofuran is distinguishable on this basis from the three other tetrachloro isomers investigated in this work. Positive ion methane chemical ionization mass spectra do show a greater degree of isomer distinction, and are reasonably reproducible. Electron attachment negative ion spectral characteristics are also presented. Preliminary results on negative ion chemical ionization mass spectra, obtained using methane plus small amounts of oxygen as reagent gas, are reported.