The amount of carbon monoxide recovered from calorimetry tests of high explosives is far larger than the amount predicted by equilibrium calculations. The present kinetics study of PETN ([(nitro(oxy)methyl]propanediol dinitrate) has revealed that the cooling of the calorimetry bomb after detonation of a PETN sample sufficiently slows those reactions that would otherwise lead to equilibrium so that these reactions are effectively frozen in the time scale of recovery of detonation products. Among these reactions, those that can create CH4 are the most important ones. Their rates are generally slow at all temperatures relevant to calorimetry tests. This and the slowing down of a reaction, CO + H2O --> H-2 + CO2, at temperatures below 1500 K are the main cause of the freeze-out of CO. A possible slow rate of the soot formation (i.e., condensed carbon) is not responsible for it. The sensitivity of the present result to the cooling rate of the detonation products and to free radicals is also examined.