Thermal decomposition of the crystalline explosive, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), is modeled using percolation theory in order to characterize thermal damage. Percolation theory has been used historically to describe fluid flow through a network of permeable and impermeable sites. To describe thermal decomposition, the permeable and impermeable sites are related to broken or unbroken bonds. For HMX, N2O groups are treated as sites connected by oxygen and methyl bridges. Bridges connect the N2O sites by C-N bonds and by intermolecular attractions between N and O. The gas-phase reaction of N2O with CH2O is also included in the mechanism. Predictions are compared to time-to-explosion data. The state of the condensed material at ignition is characterized by finite HMX-fragments of various molecular weights.