The mass spectra and gas chromatographic properties of 17 synthetic fluoro-, chloro- and bromomethoxy-biphenyls and 12 dichlorodimethoxybiphenyls have been examined. From this representative series it appears that the position of the methoxy group (ortho, meta and para to the biphenyl bond) in all monomethoxy compounds examined, and the positions of the two methoxy groups in most of the dimethoxy compounds, can be assigned unambiguously by their difference in fragmentation pattern. The value of this method was shown by metabolism experiments in which 4,4'-difluoro- and 4,4'-dibromobiphenyl were fed to rats and 4,4'-dichlorobiphenyl was administered to plants. All hydroxylated metabolites found were identified by gas chromatography mass spectrometry. Relationships between structure and gas chromatographic retention time of these compounds are discussed.