Nitrogen bases were isolated from a Brazilian diesel distillate by acid extraction. With minimum sample preparation and clean-up, nitrogen bases were analyzed by particle beam LC/MS and HPLC with photo diode-array detection using reversed phase chromatography. Benzoquinolines were identified as the major nitrogen containing compounds in this basic fraction. By using neutral mobile phases, benzoquinoline homologues were separated, enabling rapid class characterization as well as preparative HPLC isolation of individual benzoquinoline homologues. Acidified mobile phases, however, exhibited greater resolution for individual isomers. UV spectroscopy was used to differentiate various types of benzoquinolines (e.g. benzo[f]quinolines, benzo[h]quinolines, and acridines). Acridines can be easily distinguished from benzo[h]- or benzo[f]quinolines by its different UV absorption spectrum. Benzo[h]- and benzo[f]quinolines were differentiated by the differences in the first derivative of the absorbance (dA/dδ) vs. wavelength. To confirm the identification, nitrogen bases were also analyzed by conventional GC/MS methods.