Rigorous validation of a simulation model of the toe-to-heel air-injection (THAI) process has been performed using results obtained from a 3D combustion-cell experiment on Athabasca oil sands. The numerical model includes a new kinetics formulation, based on the Athabasca thermal-cracking kinetics scheme proposed by Phillips et al. (1985). The kinetics model excludes low-temperature oxidation because THAI operates in a high- temperature oxidation mode. Excellent agreement was obtained between the predicted and experimental oil-production rate, and there was generally a good match obtained for other dynamic variables, including the residual coke profile, produced oxygen, and peak combustion temperature. The numerical model provides a fundamental platform for upscaling to field scale that will enable fine-scale details of the process to be captured. Simulations were performed under dry in-situ combustion (ISC) conditions at the high air-injection fluxes used in the experiment. Under these conditions, vertical-plane temperature profiles in the sandpack confirm that the combustion front is quasivertical and forward leaning in the direction of the heel of the horizontal producer well. The shape of the combustion front was predicted more accurately from horizontal-plane profiles, showing that there was no oxygen in regions ahead of the combustion front. Oil displacement occurs mainly by gravity drainage, but pressure drawdown into the horizontal producer well below the mobile-oil zone (MOZ) is also significant. Various zones that develop during the ISC process include a steam zone located in the upstream part of the MOZ. All of the mobilized oil is produced from the MOZ, which is composed of two distinct oil regions. The first part contains oil produced by thermal cracking of the heavy residue and vaporized lighter oil. The main bulk of the oil produced in THAI comes from the second region of the MOZ, containing banked original oil. The oil that is eventually produced is partially upgraded oil because the thermally upgraded and lighter oil fractions mixing with the original oil when they enter the horizontal producer well.