Toe-to-heel air injection (THAI) is a variant of conventional in situ combustion (ISC) that uses a horizontal production well to recover mobilized partially upgraded heavy oil. It has a number of advantages over other heavy oil recovery techniques, such as high recovery potential. However, existing models are unable to predict the effect of the most important operational parameters, such as fuel availability and produced oxygen concentration, which will give rise to unsafe designs. Therefore, we have developed a new model that accurately predicts dynamic conditions in the reservoir and is also easily scalable to investigate different field scenarios. The model used a three-component direct conversion cracking kinetics scheme, which does not depend upon the stoichiometry of the products and, thus, reduces the extent of uncertainty in the simulation results as the number of unknowns is reduced. The oil production rate and cumulative oil produced were well-predicted, with the latter deviating from the experimental value by only 4%. The improved ability of the model to emulate real process dynamics meant it also accurately predicted when oxygen was first produced, thereby enabling a more accurate assessment to be made of when it would be safe to shut in the process, prior to oxygen breakthrough occurring. The increasing trend in produced oxygen concentration following a step change in the injected oxygen rate by 33% was closely replicated by the model. The new simulations have now elucidated the mechanism of oxygen production during the later stages of the experiment. The model has allowed limits to be placed on the air injection rates that ensure stability of operation. Unlike previous models, the new simulations have provided better quantitative prediction of the fuel laydown, which is a key phenomenon that determines whether or not successful operation of the THAI process can be achieved. The new model has also shown that, for completely stable operation, the combustion zone must be restricted to the upper portion of the sand pack, which can be achieved using higher producer back pressure.