A theoretical equation of state for detonation products is described and compared with various data. A perturbation theory approach is used for the mixture of molecular fluids that is based on Monte Carlo simulations. The solid carbon is characterized as small clusters rather than as a bulk material. The free energy of the clusters is modeled as primarily additive contributions such as vibrational modes, bond strengths, and effective volumes. Up to 30&percent; of the atoms in a diamond cluster are on the surface with dangling bonds capped by various groups composed of C, H, N, and O from the background fluid. A counting term similar to ideal entropy of mixing is also found for the surface composition. Competition between the U, TS, and PV terms in the Gibbs free energy leads to dramatic shifts in the surface composition in some regions. This in turn leads to shifts in the background fluid mixture composition and anomalous behavior in the total EOS. This behavior in PBX 9502 is in good agreement with recent data on release isentropes from overdriven states (1). ©2000 American Institute of Physics.