The transient behavior of confined HMX in response to an external heat flux has been simulated using a detailed kinetic model. The model is spatially one-dimensional, fully transient, and consists of equations for modeling the solid (condensed) phase HMX, the gas phase, and the surrounding container for fast cook-off conditions. The condensed-phase decomposition reactions are described by semi-global, distributed kinetics. The gas phase description includes a detailed chemistry model for the combustion of HMX. A nonlinear, numerical solution of the partial differential equations results in predictions of temperature, pressure, velocity, and species fractions as a function of position and time. Predicted time-to-ignition yields good agreement with experimentally measured fast cook-off times. Temperature at ignition is found to be a function of heating rate. The ignition process for fast cook-off is consistent with laser ignition of bare HMX samples. Simulation results depend on accurate boundary conditions applied to the steel container. Pressurization rates are calculated and insight is given into the fast cook-off behavior of HMX. The main decomposition pathway is the conversion of HMX into CH2O and N2O. Published by Elsevier Inc. on behalf of The Combustion Institute.