Athabasca bitumen residuum was reacted under nitrogen, hydrogen, or hydrogen in the presence of a residuum hydrotreating catalyst for varying times which produced between 40 and 90 % residuum conversion. There was little difference among the relative rates of conversion for the three series. The data suggest that the first 30-40 % of residuum conversion mainly involves breaking of labile carbon-to-carbon bonds to produce distillate. It is suggested that additional conversion results from the formation of an aromatic-carbon aliphatic-carbon biradical intermediate, which is formed from hydroaromatic structures. Hydrogen transfer to the aromatic-carbon radical center is sufficient to prevent condensation leading to coke formation. The hydrogen radical produced as part of the hydrogen-transfer process can attack a condensed aromatic center, leading to gas and distillate formation from large aromatic molecules. The initially formed aliphatic-carbon radical can preferentially undergo fragmentation to produce gas and distillate.