The thermal cracking of PR Spring bitumen-derived heavy oils in the presence of hydrogen was studied in a fixed-bed reactor as a function of process operating variables. The reactor was filled with a catalytically inert sodium exchanged alumina. The packing consisted of an alumina hydrodenitrogenation catalyst support which had been impregnated with 2 wt.% sodium to reduce the acidity of the alumina. Thermal reactions were investigated with regard to denitrogenation, desulfurization, demetallation, Conradson carbon residue (CCR) reduction and residuum conversion. The process operating variables investigated were temperature (642-683 K), and WHSV (0.25-0.74 h(-1)). The reactor pressure and hydrogen-to-oil ratio were fixed at 13.7 MPa and 890 m(3)/m(3) (5000 scf H-2/bbl); respectively, in all experiments. The product distributions and yields were also determined as a function of process operating variables. Residuum conversion over the sodium-impregnated catalyst support was greater in all cases relative to nitrogen and sulfur removal due to thermal cracking of the residuum. Only low levels of heteroatom conversion were possible via thermal reactions. The extent of nitrogen removal over the sodium-impregnated catalyst support was lower than that of all other classes of component-types. Significant levels of CCR conversion were obtained despite the absence of metal sulfides on the alumina and were attributed to hydrothermal conversion.