Abstract Gas chromatographic–mass spectrometric analyses of alkylbiphenyls and alkyldiphenylmethanes have been performed on a series of crude oils reservoired in the Gippsland Basin (Australia), as well as in some marine coastal sediments contaminated with a condensate. Changes in the isomeric distribution of the alkylbiphenyls and alkyldiphenylmethanes with increasing biodegradation were investigated. The relative abundance and distributions of the isomers were found to change significantly with increasing biodegradation. Isomers with the 4-substituent(s) generally increase in abundance relative to isomers that lack this feature. Thus, alkylbiphenyl and alkyldiphenylmethane isomeric distributions are useful indicators of the extent of petroleum biodegradation in reservoirs and surface sediments. It is suggested that geosynthetic processes and/or relative susceptibilities of certain isomers to microbial attack can account for changes in the isomeric distribution of these components with increasing biodegradation. Significant differences in the relative abundances of the methylbiphenyls are apparent and are probably due to variations in the conditions that the petroleum had been subjected to in the two contrasting environments.