Nanosecond and picosecond laser irradiations have been used to study the decomposition of ammonium perchlorate (NH4ClO4) crystals, a main component of propellants. Chlorate (NH4ClO3) decomposition product was detected via X-ray photoelectron spectroscopy. The decomposition is initiated amid associated mechanical deformations and microcracking processes occurring on a time scale commensurate with actual frequencies of energetic crystal decompositions pertinent to propellant combustion. Optical, scanning electron and atomic force microscopy methods have been applied to characterization of the laser-damage zones. Individual initiation or residual "hot spot" sites have been detected in the electron and atomic force microscope images, and are related to the cracking behaviour of the perchlorate allotropic phases. Evidence of the 240 degrees C orthorhombic to rock-salt type cubic transformation was obtained in nanosecond laser irradiations through a remnant microstructure of ultrafine cracks whose intersection points marked an array of decomposition sites. A dislocation model description is given for the connected cracking and decomposition site observations.