High pressure accelerating rate calorimetry (ARC) tests have been performed on three different crude oils at their respective reservoir pressures. The experiments on medium heavy Clair oil and heavy Wolf Lake oil used clean silica sand, incorporating 3% kaolinite, to represent the reservoir matrix, whereas Athabasca Tar Sand was used in its preserved, virgin state. The Clair oil and Wolf Lake oil tests involved high initial water saturations in the reservoir, representative of a post-waterflooded or a post-steam injection state. A second test on Athabasca Tar Sand used a lower oil (bitumen) saturation, but without an adjustment of the original brine saturation. Although the medium Clair oil exhibited high exothermicity throughout the temperature range, the temperature detected for the onset of low temperature oxidation (LTO) was much higher than that for Athabasca Tar Sand. The calculated activation energies also indicate that Athabasca Tar Sand is very reactive in the LTO region. Furthermore, the overall continuity exhibited by the measured exotherms indicates that all three oils are potentially good candidates for in situ combustion as an oil recovery method.