The activity of promoted molybdenum on gamma-alumina catalysts toward residuum, Conradson carbon residue (CCR), and heteroatom conversions and solids formation in the reactor during hydrotreating and hydrocracking of Athabasca bitumen has been investigated during the first day on stream by using batch reactors and for up to 30 days on stream using a continuous flow pilot plant. It is proposed that, under hydrotreating conditions, catalytic CCR conversion and catalytic metals removal take place on different active sites from catalytic sulfur removal and that catalytic nitrogen removal takes place on different sites again. The active sites on the catalyst which promote conversion of CCR and heteroatoms through conventional hydrogenation reactions were lost in a matter of hours under cracking conditions. After the hydrogenation sites were deactivated, the catalyst remained active toward CCR and sulfur removal, presumably through a different mechanism. CCR conversion was strongly correlated with residuum conversion and initially there was no selectivity between conversion of CCR residuum molecules and non-CCR residuum molecules. Selectivity was introduced as the catalyst deactivated over 30-50 days with CCR conversion decreasing faster than residuum conversion. It is proposed that the main role of catalyst during hydrocracking is to assist in the reaction of thermally generated phenyl radicals with molecular hydrogen, resulting in the addition of a hydrogen atom to condensed aromatic centers, and ultimately resulting in the decomposition of the condensed aromatic unit to give gases and distilate. Hydrogenation of aromatics does not play a significant role in the hydrocracking of Athabasca bitumen residuum. The catalyst was only indirectly involved in preventing solids formation in the reactor during hydroprocessing.