Aquathermolysis experiments were performed on core samples taken from three large bitumen and heavy oil deposits found in Alberta, to investigate gas evolution over the temperature range 360 to 420-degrees-C. Experiments conducted on Athabasca included runs with an initially pre-oxidized oil sample and runs with a change in core mineralogy. Pre-oxidizing the oil was found to substantially increase the amount of carbon monoxide and molecular hydrogen generated. Core mineralogy played an important role in the generation of carbon dioxide, and the amount of hydrogen sulphide produced was dependent on oil composition, mineralogy and time. Although substantial amounts of gaseous products are produced by simple thermolysis reactions (i.e., without water present), the main thermal recovery methods, steam injection and in-situ combustion, bring the oil phase and its host rock into direct contact with water. As water has been shown to take part in thermal cracking reactions, these experiments provide usful data for the estimation of produced gas composition during thermal recovery projects.