This paper presents the results of experimental and computational studies that were carried out to determine the critical depressurization rate for the extinction of ammonium perchlorate (AP) monopropellant. AP monopropellant burning at a pressure of 70 bar was depressurized to a final pressure of 25 bar, experimentally and critical depressurization rates were determined. The computational model chosen is two-dimensional in nature which couples both the gas and condensed phase and simulates the burning of an AP pellet, which had been compared extensively with experimental results available in the literature. It is found through the numerical simulations that the condensed phase plays an important role in extinction by rapid depressurization. A large amount of convective heat loss associated with the experiments was also simulated using a rudimentary convective heat loss model in the condensed phase. The critical depressurization rate determined experimentally for AP monopropellant is found to lie between 2700 and 3000 bar/s for 4.5 mm thick pellet with heat loss. The computed value for the same condition is 5000 bar/s. This indicates that there is a good match between experiments and computations. © 2017 The Combustion Institute