A microscopic model is developed to describe the kinetics of thermal decomposition of microcrystals taking place on the crystals' surface, and the model is used to interpret certain experimentally observed decomposition rate (and product-formation) data of high-energy crystalline materials such as solid HMX. This so-called sponge model does not invoke autocatalytic steps yet is consistent with phenomenologically observed rate laws that display rapid rate enhancements early in the decomposition process. Its most essential assumptions are that (1) a molecule on the surface of a microcrystal undergoes unimolecular decomposition to form products (mainly gases) that promptly vacate the neighborhood of the just-dicomposed molecule and (2) a molecule lying within the microcrystal (i.e., not on its surface) experiences packing forces from the surrounding crystal molecules that essentially prohibit its unimolecular decomposition. These assumptions, when incorporated into a kinetic model, allow the microcrystals to develop a spongy character (which the experimental data display) as their surface molecules decompose and expose underlying solid material thus generating new reactive molecules as the surface-area-to-volume ratio grows. It is this growth in surface area that produces the rate enhancement in the microcrystals' early stages of decomposition.