To characterize and model the combustion of a reactant mixture in a spark-assisted compression ignition (SACI) engine, one-dimensional reaction front propagation into end-gas mixtures with varying degrees of reaction progress is simulated using a skeletal iso-octane mechanism with variable transport properties. The dominant mechanism for the end-gas auto-ignition and combustion is identified based on a ratio of the corresponding flame to homogeneous ignition time scales, as a means to distinguish the transport-controlled and chemistry-controlled combustion regimes. The results indicate that reaction fronts propagating into end-gases are deflagrative provided that the temperature at the reaction front base is below 1100 K, while beyond this temperature, transport has little effect on the one-dimensional solution, indicating that reaction front propagation is chemistry-controlled. The results suggest that reaction front combustion regimes are strongly influenced by and can be separated with the end-gas temperature at the base of the reaction front.