This report is concerned with the mechanisms that govern the pressure depedence of the burning rate of ammonium perchloratehydrocarbon binder solid propellents, with emphasis on propellants with bimodal ammonium perchlorate particle size distributions that tend to exhibit “plateau” and “mesa” burning. The microscopic details of the flame complex, the ingredient decomposition, and the burning surface configuration are considered as three dimensionally coupled, pressure dependent processes. New experimental results are presented concerning these processes and how they depend on pressure. No attempt is made to produce a complete model of plateau burning; however, useful guidance is provided toward that goal. © by E. W. Price, S. R. Chakravarthy, R. K. Sigman, and J. M. Freeman.