Fractions from Athabasca bitumen that had been hydrocracked at 430 degrees C and 13.9 MPa were analyzed for total sulfur content and organic sulfide (or thioether) content. The sulfides were oxidized to sulfoxides and then measured semiquantitatively by infrared spectroscopy. Thiophene-type sulfur was estimated by subtracting the sulfides from the total sulfur. After catalytic hydrocracking over Ni-Mo on gamma-alumina, the sulfur in the distillate fractions was predominantly thiophenic. The sulfur remaining in the residue fraction, however, was mainly in sulfides, and the proportion of sulfides increased with reaction severity. The fraction of sulfides was reduced when no catalyst was present and when the hydrogen pressure was reduced. This behavior was consistent with a simple kinetic model wherein sulfides were formed as intermediate species by hydrogenation of thiophenes. This type of reaction network, involving series and parallel reactions, can give apparent reaction orders for overall desulfurization ranging from negative values to greater than 2, depending on the extent of conversion and the relative rate constants.