A detailed master equation simulation has been carried out for the thermal unimolecular decomposition of CsHlo in a shock tube. At the highest temperatures studied experimentally [J.H. Kiefer and J . N . Shah, J . Phys. Chem., 91, 3024 (198711, the average thermal vibrational energy is greater than the reaction threshold and therefore (AE) (up and down steps) is positive for molecules at that energy, rather than negative; the converse is true at lower temperatures. The calculated incubation time, in which the decomposition rate constant rises to I / e of its steady state value, is found to be only weakly dependent on temperature (at constant pressure) between 1500 K and 2000 K and to depend almost exclusively on ( A E ) , (down steps, only), and not on collision probability model. Simulations of the experimental data show the magnitude of ( A E ) , depends weakly on assumed collision probability model, but is nearly independent of temperature. The second moment (AE)'" is found to be independent of both temperature and transition probability model. The experimental data are not very sensitive to the possible energy-dependence of ( A E ) , for a wide range of assumptions. It is concluded that the observed experimental "delay times" probably can be identified with the incubation time; further experiments are desirable to test this possibility and obtain more direct measures of the incubation time.