The minimum identifiable quantities of 55 toxic substances have been determined by packed column gas chromatography/Fourier transform infrared spectroscopy (GC/FT-IR) at conditions compatible with environmental extract analysis. Identification of each GC effluent component was accomplished with an IR reconstructed chromatogram, subsequent interferogram transformation, and on-line library search. GC/FT-IR exhibited its greatest sensitivity to aliphatic and aromatic organic compounds containing carbonyl or other oxygenated functional groups and its poorest sensitivity to alkyl halides and aromatic hydrocarbons. GC/FT-IR sensitivity was inversely proportional to the light pipe temperature. Extracts of the residue from a chemical plant still bottom were injected into the GC/FT-IR system, and the separated components were identified by utilizing an infrared reconstructed chromatogram (IRC) to locate interferogram files of high signal-to-noise ratio. Deconvolution techniques were used to separate the contributions of co-eluting IRC peaks. Interferogram files were then co-added and transformed. An on-line library search of 2300 vapor-phase infrared spectra provided sufficient chemical functionality information to identify most of the extract components.