We have analyzed eight heavy vacuum gas oil (HVGO) distillation fractions, initial boiling point (IBP)-343, 343-375, 375-400, 400-425, 425-450, 450-475, 475-500, and 500-525 degrees C, of an Athabasca bitumen by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Acidic, basic, and nonpolar components were detected by negative-ion and positive-ion clectrospray ionization (ESI) and automated liquid injection field desorption ionization (LIFDI) positive-ion FT-ICR MS. Ultrahigh mass resolving power (m/Delta m(50%) approximate to 350 000) and high mass accuracy (<500 ppb) facilitate the assignment of a unique elemental composition to each peak in the mass spectrum. Thus, each distillate was characterized by mass, heteroatom class, type (number of rings and double bonds), and carbon number distribution to correlate compositional changes with increased boiling point. Negative-ion ESI FT-ICR MS identifies high relative abundance nonaromatic O-2 species that span the entire distillation range. All ionization methods reveal an increase in double-bond equivalents (DBE, the number of rings plus double bonds) and carbon number with increased distillation temperature. In addition, some structural information can be inferred from increases in DBE value with increased distillation temperature. Summed data for individual distillation cuts yield class specific isoabundance contours similar to that for the feed HVGO, suggesting that class-specific carbon number and DBE distributions for individual distillation cuts could be estimated from the high-resolution feed HVGO mass spectrum.