The anomalous deflagration behavior of ammonium perchlorate (AP) was analyzed at elevated pressures. The deflagration rate of pure AP exhibited a substantial mesa burning trend in the 2000-4000 psi pressure range. The samples of both single crystals and pressed pellets of AP used in the study were nominally in the shape of rectangular laminas. The thermally quenched samples were found to exhibit anamalous surface features at pressure below the onset of mesa burning rate trend of pure AP. © 2002 American Institute of Aeronautics and Astronautics, Inc.