While burning at subatmospheric pressure, composite propellants generally fume and under certain conditions eject partially reacted particles. This characteristic property is exploited in this study to understand the mechanism of burning rate modification. The additives CuCr204, Fe203, Sb2 03, AI(OH)3 gel, and LiF were used with unmetallized AP/CTPB (carboxy terminated polybutadiene) composite propellants, and the following studies were done: (1) burning rate measurements below and above atmospheric pressure (up to 70 bar), (2) quantitative chemical analyses of fume deposits and ejected particles, and (3) scanning electron microscope examination of the microstructure of the surface of extinguished propellant samples. The results of the studies at subatmospheric pressure indicate the following: (1) burning rate enhancers augment the AP/binder interracial reactions, (2) CuCr20+ decomposes into copper oxide and chromium oxide with net heat release, and if this occurs at the surface, it constitutes one route by which the burning rate could be enhanced, (3) FeaO3 possibly enhances near-surface breakdown of heavy fuel molecules, and (4) the melt layer on AP particles and the low-viscosity binder-melt, which exist when LiF is present in the propellants, possibly depress the burning rate.