Untreated and hydrotreated nitrogen-concentrated distillation fractions (483-504 degrees C) from Syncrude oil have been analyzed by electron ionization, field desorption ionization, and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The routinely attainable mass resolving power >= 300 000 at m/z 300 and sub-ppm mass accuracy allowed for resolution and elemental compositional assignment of various compound classes. Multiple closely spaced mass doublets ( with mass differences < 10 mDa) were detected. Although hydrotreatment reduced the relative abundances of several heteroatomic classes (N-2, S-3, NS2, NOS, O-3, OS2, and O2S) below detectability, N-, S-, O-, O-2-, OS-, and NO-containing compounds survived hydrotreatment. Analysis of N-, S-, and O- containing compounds and hydrocarbon compounds at the molecular level exposes compositional changes and reveals the effectiveness of the hydrotreating process.