Heavier oils are becoming increasingly important as a refinery feedstock. In order to optimize quality and yield of products from the vacuum residues of these intractable feeds, more information on their molecular chemistry is required. Super-critical fluid extraction and fractionation (SFEF) with pentane provides the opportunity to cut deep into the bottoms while readily yielding enough of each fraction to allow characterization and reactivity studies. Results show that problematic species and SARA components (saturates, aromatics, resins and asphaltenes) are asymmetrically distributed among the separated fractions. The determination of average structures based on bulk analysis of the whole residue is heavily biased towards the lighter fractions and provides little information on heavy, difficult to process components. A comparison of bitumen pitch with residues from benchmark crudes, heavy oils and bitumen from non-Athabasca sources, demonstrates that it is not significantly different in its chemical make-up or reactivity to other hydrocarbons. Compared to conventional oils, the intractable nature of bitumen can be largely ascribed to its much higher asphaltene and contaminant contents. (C) 2007 Elsevier B.V. All rights reserved.