AbstractThirty seven of the thirty eight isomers of tetrachlorodibenzofuran (TCDF) have been synthesized by unambiguous routes, and purified. The 1,2,8,9‐TCDF isomer could not be thus prepared, and attempts to make it by pyrolysis of appropriate precursors also failed. Retention characteristics of TCDFs under gas chromatographic conditions were determined, and found to agree well with those predicted by an existing mathematical model derived from data obtained for TCDF isomers prepared by more ambiguous procedures. Mass spectral characteristics were also obtained, particularly relative response factors for the molecular ions of the TCDF isomers under electron ionization (EI) conditions. A discussion of the likely transferability of these relative response factors between mass spectrometers of different types is included. Chemical ionization (CI) mass spectra were also obtained, for both positive and negative ions. These CI spectra yielded little structural information not available from the EI data, and problems of lack of sensitivity and of reproducibility render them less suitable for trace analysis of TCDFs. Some unusual ion chemistry was observed under CI conditions, which can be rationalized in terms of pre‐ionization attack on the TCDF molecule by free radicals in the sou