Hydrogen abstraction from compounds such as benzene and biphenyl is a potential complication when these aromatics are used as diluents in hydrocarbon pyrolysis experiments. This paper presents a general methodology for quantitatively assessing the likelihood that substrate-derived radicals abstract hydrogen from a nominally inert diluent. The key variables are the concentration of the substrate, the pyrolysis temperature, and the dissociation energies of the C-H bonds attacked. Hydrogen abstraction from an aromatic diluent becomes more important as the temperature increases, the substrate concentration decreases, and the C-H bond dissociation energy of the substrate increases. All other factors being equal, the relative rate of hydrogen abstraction from an aromatic diluent is about 10 times higher during the pyrolysis of an n-alkane than during the pyrolysis of an n-alkylbenzene.