Hydrotreating (HT) kinetics of Athabasca. bitumen-derived gas oil has been studied between 340 to 420 C using a commercial NiMo/gamma-Al2O3 catalyst. The kinetics analyses included overall conversion of high-boiling species into low-boiling products, hydrodenitrogenation (HDN) of total, basic and non-basic nitrogen compounds and hydrodesulfurization (HDS). Three temperature regimes were marked out for the kinetic analyses: low (340-370 degreesC), intermediate (370-400 degreesC) and high (400-420 degreesC). The mechanism for the conversion of high to low-boiling species was observed to change from one temperature regime to the other, giving rise to different activation energies. HDS and HDN activation energies increased in the order: high < low < intermediate severity temperature regime.