In situ combustion is a possible method for producing heavy oil when other methods such as SAGD are not adequate (e.g., in thin beds, or when CO2 emission for steam generation is unacceptable). Previous field trials of this process have often been, unsuccessful. However, in recent years, several new well implementations have been proposed (COSH, THAI), exploiting the more advanced drilling capabilities now available. Simulations of such configurations require a reliable representation at field scale of the oxy-combustion reactions, which is not available at; the present time. The objective of the work described in this paper is to illustrate some improvements in the description of oxy-combustion reactions both at the experimental level and in the simulation. models. A "ramped temperature" experiment has been conducted, on an extra heavy stock tank oil (10,000 cP at reservoir conditions). This experiment has been successfully matched using a commercial simulator. The improvement over classical adiabatic reactor experiments is significant: two combustion reactions are clearly observed, and the Arrhenius parameters are determined with increased accuracy. The reliability of the inferred parameter values is checked by applying them to simulations of previous adiabatic disk reactor experiments conducted under a variety of conditions. The final part of the paper is dedicated to illustrating the impact of the new reaction scheme on the simulation results at field scale.