Bitumen derived gas oils have unique properties and are difficult to hydrotreat. These gas oils also significantly deactivate conventional catalysts; new, alternative catalysts have shown only limited improvements. The key for process and quality improvements is a better understanding of the molecular chemistry of gas oil. The approach applied in this work involves a combination of gas oil fractionation by high performance liquid chromatography (HPLC) and analysis of the resulting fractions by advanced spectroscopic methods. Results show that the HPLC saturate fraction is practically free of aromatic components. The nominal mono- and di-aromatics fractions differ in that the former has a higher molecular weight and H/C ratio but lower aromaticity and heteroatom content. The HPLC three poly-aromatic sub-fractions comprise molecules with an average of only two aromatic rings. Also, the H/C ratio and aromaticities of these samples are virtually the same. Regardless of their structural resemblances they differ substantially in their HPLC elution behaviour. This can be attributed to the differences in their oxygen and, especially, nitrogen contents. The asymmetric distribution of nitrogen may allow its selective removal thus opening the possibility for less expensive treatment, especially in terms of catalyst utilisation. Crown Copyright (C) 2004 Published by Elsevier Ltd. All rights reserved.