The burning rate trends of both pure binder sandwiches and those containing a mixture of binder with fine ammonium perchlorate particles are investigated over appreciable ranges of pressure and thickness of the middle lamina in the sandwich. The crucial aspect of this study is the wide variation of the fine ammonium perchlorate/binder ratio in the middle lamina of the sandwiches. The sandwiches with fine ammonium perchlorate particles have been quenched by rapid depressurization and their surface profiles have been carefully mapped for different conditions. The gas phase flame structure is estimated through numerical simulation, with the observed surface profiles and burning rates taken as inputs. It is found that, as the fine ammonium perchlorate particles are added to the binder in small quantities, they tend to dilute the fuel and increase the lamina thickness for maximum interaction between the leading edge flames, but the trend in the variation of the optimum thickness for maximum burning rate with increase in pressure is reversed when compared to the case of pure binder sandwiches reported earlier. The burning rates of these sandwiches decrease slightly relative to those of the pure binder sandwiches. With higher fine AP content, there is greater interaction between the leading edge flames due to the inward shift of the stoichiometric surface and the extension of the fuel-rich sides of the leading edge flames over the middle lamina, ultimately resulting in the establishment of the canopy premixed flame between them. This restores the optimum thickness trends with pressure as in the case of pure binder sandwiches. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.