The combustion behavior and thermochemical properties of two types of RDX-based composite propellants, XM39 and M43, are studied and compared in order to investigate the effect introduced by an energetic ingredient in the M43 propellant formulation. Due to the enhanced rate of heat release on the burning surface caused by the energetic material of M43 propellants, the burning rate of M43 propellants is found to be higher than that of XM39 propellants. The presence of the energetic material in the M43 propellant formulation also increases the burning surface temperature and thermal diffusivity of M43 propellants; it does not, however, introduce any observed changes in the flame structure and the heat feedback from the gas phase to the condensed phase. Based upon SEM pictures, burning-surface structures of the recovered XM39 and M43 propellant samples are found to be significantly different, indicating the strong effect of the energetic material on the surface reaction. A set of important thermochemical properties of XM39 and M43 propellants is deduced from the experimental data.