The objective of this work has been to characterize the nature of the primary exhaust from fuel-rich boron-containing ca. 50 rocket motors, and to study its combustion. Two formulations were studied, each at two pressures. Solid materials collected from motor firings were examined microscopically, analyzed chemically, and studied for ignition temperatures and burning times, Tb. Both the theory and the experiment show that boron undergoes substantial chemical reaction in primary combustion. Both soluble and insoluble boron compounds form.