Conventional in situ combustion (ISC) has largely failed to establish itself as a major heavy oil recovery process. However, the recent successful field piloting of the toe-to-heel air injection (THAI) process by Petrobank, in the Athabasca oil sands, Conklin, Alberta, Canada, seems set to develop a major new era of ISC technology. A new numerical model of the THAI-ISC process has been developed, using three-dimensional combustion cell data in conjunction with Phillips thermal cracking kinetics. Excellent agreement was obtained between the simulation predictions and a number of measured dynamic parameters, including the oil production rate. The fine-scale predictions have enabled new insights into the operation of the process, especially concerning the important mobile oil zone (MOZ) from which partially upgraded oil is produced. The properties of the MOZ not only affect oil production but also determine to what extent further in situ upgrading of the bitumen is possible using the catalytic THAI-CAPRI process.