The thermal decomposition of Athabasca bitumen asphaltenes was investigated in the temperature range 350 to 430 degreesC. The cracking kinetics of the asphaltenes and their intermediates were analyzed on a total a molar basis, to avoid the assumptions inherent in lumped kinetic models. The apparent first-order activation energy was 176 kJ/mol over the temperature range. Reaction selectivity changed from evolution of hydrogen sulfide to evolution of hydrocarbon gases and liquids over the same range of temperature. This change was consistent with a shift of the controlling reaction mechanism from the cleavage of C-S bonds to the cleavage of C-C bonds as temperature increased past 400 degreesC. The formation of hydrocarbon gases was the dominant reaction on a molar basis at temperatures over 400 degreesC, therefore, these reactions require more attention in mechanistic models for cracking of heavy petroleum fractions.