The plateau burning behavior of composite solid propellants consisting of ammonium perchlorate (AP) and hydrocarbon (HC) binder with a bimodal AP particle size distribution (coarse and fine) is examined. The focus is the weak pressure dependence of the propellant burn rate (i.e., a plateau) in an intermediate range of about 2.7-6.9 MPa (~400-1000 psi). The relationship between the appearance of this mid-pressure plateau for a composite propellant and self-extinction during the burning of the corresponding fine AP/binder matrix (i.e., the propellant formulation without the coarse AP particles) is experimentally examined through the study of a compositional array of propellants, sandwiches (two-dimensional propellants) and matrixes. The burning history of the samples was captured with a highspeed digital camera, and surfaces from quenched samples (burning that was self-extinguished or intentionally interrupted) are analyzed using a scanning electron microscope. The combined results indicate the prevalence of intermittent burning of the matrixes as the pressure is varied across the boundary between continuous burning and self-extinction (burn/no-burn boundary). The burning surfaces are marked by extreme threedimensionality coupled with a redistribution of the fine AP particles and the binder. The results point to the need for a more realistic approach to the underlying processes that contribute to plateau burning rate trends in bimodal composite propellants than has been adopted hitherto. © 2001 by S. R. Chakravarthy, J. M. Seitzman, R. Lillard, E. W. Price, and R. K. Sigman.