The relative magnitude of catalytic and thermal reactions during the hydrotreating of PR Spring bitumen-derived heavy oil was evaluated in a fixed-bed reactor as a function of temperature, residence time and catalyst selection. The relative effects of thermal and catalytic reactions were evaluated by hydrotreating the PR Spring bitumen-derived heavy oil over three catalysts: sulfided Ni/Mo/alumina HDN catalyst, Mo/alumina HDM catalyst and sodium-impregnated alumina. Catalytic and thermal effects for each of the catalysts were evaluated under a range of temperature (625-685 K) and liquid hourly space velocity (0.14-0.81 h(-1)). The reactor pressure, 13.7 MPa, and the hydrogen-to-oil ratio, 890 m(3) m(-3) (5000 scf H-2 per bbl), were fixed in all experiments. The catalysts＇ activities were ranked as follows: HDN catalyst > HDM catalyst > sodium-impregnated alumina, based on their activities for nitrogen, sulfur and nickel removal as well as for the conversion of Conradson carbon residue (CCR) and residuum. The catalyst activities were strongly dependent on the metal loading and were dependent, to a lesser extent, on the acidity of the alumina support. CCR and residuum conversion was closely linked relative to heteroatom and metal removal. This is attributed to a significant overlap between moieties which are classified as CCR precursors and moieties which are classified as residuum. Catalyst selection significantly affected residuum conversion. This is because the high catalyst densities employed in packed bed reactors accentuate catalytic reactions relative to thermal reactions. Although sulfur was generally more reactive than nitrogen, sulfur conversions in excess of 70% were difficult to achieve. It was presumed that 30-40% of the sulfur was asphaltic in nature and exhibited low reactivity. For this reason it was easy to achieve significant sulfur conversion as low severities, but difficult to achieve deep desulfurization at high severities. The deactivation rates of HDN and HDM catalysts were 0.06 and 0.2 degrees API per day, respectively.