Seven gas oil (GO) cuts from the same atmospheric petroleum residuum were obtained by molecular distillation at final temperatures of 490.0 and 503.2 degrees C (medium GO), 522.5 and 549.5 degrees C (heavy GO), and 583.7, 622.4, and 662.2 degrees C (extra heavy GO). The detailed chemical composition of these samples was investigated using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC X GC-TOFMS) and electrospray ionization ultrahigh-resolution and -accuracy Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). Many compound classes were identified by GC x GC-TOFMS, such as tri-, tetra-, and pentacyclic terpanes, steranes, and secohopanes; several polycyclic aromatic hydrocarbons (PAHs), such as fluorene, phenanthrene, pyrene, and benzo[g,h,i]perylene; sulfur compounds, e.g., alkylbenzothiophenes, alkyldibenzothiophenes, and alkylbenzonaphthothiophenes; and alkylphenols. With ESI FT-ICR MS in both the positive- and negative-ion modes, many polar components that cannot be directly characterized via GC were identified. These included quinolines, benzoacridines, carbazoles, benzocarbazoles, thiophenequinolines; and also furanacridines and trends in carbon number and unsaturation as measured by the double-bond equivalent number (DBE) were monitored. Comprehensive characterization of the chemical composition was therefore possible using these two powerful and complementary techniques, enabling detailed monitoring of the complex chemical composition of GO cuts and its variation as a function of distillation temperatures.