In heavy oil production by in situ combustion, information on the relative importance of the combustion and steam fronts is very useful in the development of good production strategies. To obtain this information for two reservoirs, one containing heavy oil and the other a bitumen, a novel series of combustion tube tests was conducted. Contrary to conventional belief, the results showed that, before steam breakthrough, the produced oil properties were influenced much more by steam distillation than by Cracking reactions. While the combustion front mobilized substantial amounts of oil, almost all of it remained behind the steam-front. This oil displayed significance differences in properties and composition compared with the original oil. Overall, the results helped to provide some guidelines for developing Improved strategies for fireflood operation, and emphasize some requirements for the prediction of fireflood behaviour.