Comprehensive two-dimensional gas chromatography (GC × GC) was used to explore and compare the chemical complexity of oil released from the Deepwater Horizon (DWH) disaster in 2010 and the Ixtoc I spill in 1979-1980, both in the Gulf of Mexico (GoM). To provide the most complete inventory of the compounds present in the DWH and Ixtoc I crude oils, we utilized GC × GC systems coupled to a flame ionization detector and a high-resolution time-of-flight mass spectrometric detector. The results of this study demonstrate the significance of valuable environmental forensics information obtained using GC × GC fingerprinting methods. In particular, the high-resolution mass spectrometer enabled an in-depth characterization of the types and families of GC-amenable compounds present in these crude oils including the detection of highly alkylated sulfur-containing species, alkylated carbazoles and benzocarbazoles, and a suite of unusual de-A-sterane biomarkers in the Ixtoc I oil. This type of specificity is essential for differentiating spill sources of similar origin/type, for example, within northern and southern GoM petroleum families and of the molecular transformations that occur during oil-spill weathering processes.