The electron impact mass spectra of various methoxyhalobiphenyls have been reinvestigated. Previous findings, concerning the effect of the methoxy group upon the main fragmentation routes, have been confirmed and extended. In addition, studies of unimolecular and collision induced fragmentation of ions, using linked scan techniques, have been made. Such an approach permits the identification of substances with a minimal chromatographic separation prior to mass spectrometry. It has also led to some clarification of the fragmentation pathways leading to the observed cracking patterns.