Research described in this paper was conducted in support of a more extensive study that has been ongoing at the University of Calgary to quantify the effect of the presence of carbon dioxide and oxygen mixtures in an Athabasca bitumen reservoir undergoing a carbon dioxide flooding/sequestration process. This paper concentrated on the compositional and rheological changes of Athabasca bitumen. The experiments were performed in the oscillating batch reactor using clean oil plus distilled water only. Viscosity was measured, and the compositional data were expressed in terms of the components maltenes, asphaltenes, and coke. In CO2/O-2 tests, the amounts of maltenes decreased with increasing reaction temperatures as a result of conversion to asphaltenes and coke. The data had direct applicability to recovery processes involving the injection of carbon dioxide containing oxygen as an impurity in the bitumen reservoir.