The ortho effect observed for chlorinated biphenyls (PCBs) and brominated biphenyls (PBBs) having 2,2; 2,2', 6 or 2,2', 6,6' halogens, can be combined with GC retention index for isomer specific identifications by gas chromatography/mass spectrometry (GC/MS). This technique reliably allows accurate structure assignments for environmental monitoring without the use of multiple GC determinations that employ different chromatographic phases. Validation of the ortho effect for 129 PCBs and PBBs is reported, including a survey of all PCB mass spectra in the EPA/NIH mass spectral database. Presence or absence of ortho effect and reduction in magnitude for 2,2', 6,6' substitution may be related to the formation of a planar, bridged halonium ion intermediate upon electron ionization in the mass spectrometer. (Copyright (c) by John Wiley & Sons, Ltd.)