High-resolution (100 000 < M/Deltam(50%) < 250 000, in which Deltam(50%) denotes mass spectral peak fall width at half-maximum height) electrospray ionization Fourier transform ion cyclotron resonance positive-ion mass spectra of unprocessed (and processed) diesel fuels resolves approximately 500 (and 200) chemically different constituents over a mass range from 200 to 452 Da, with as many as 6 resolved elemental compositions at a given nominal mass. Molecular formulas were assigned from accurate mass measurement to within +/-1 ppm. Compound types were identified by Kendrick mass analysis. On the basis of the experimental behavior of model compounds, electrospray ionization was found to ionize selectively basic pyridine homologues-compounds responsible for deactivation of hydrotreatment catalysts and instability of fuels during storage. Compound classes identified in the unprocessed diesel fuel include those containing N, N-2, NS, NO, N2O, O-2, and SO and, in the processed diesel, N, N-2, NO2, and SO. Comparison of unprocessed and processed diesel fuel reveals that N- and N-2-type compounds are resistant to laboratory hydrotreatment. In contrast, NS-, NO-, N2O-, and O-2-containing compounds were completely removed by hydrotreatment. Concentration-dependent dimers observed above 450 Da were confirmed by infrared multiphoton dissociation.