Improving the neutralization of bacterial spores is of paramount importance for the bioterrorism defeat. In this study, we investigate the synergism between rapid heating (similar to 10(4) degrees C/s to similar to 10(5) degrees C/s) and chlorine gas in the neutralization of Bacillus thuringiensis (Bt) spores - a close relative of Bacillus anthracis (Ba), which is a known biowarfare agent. Bt spores were heated in a gas chamber with defined concentrations of Cl-2 gas and relative humidity (RH). The critical peak temperature (T-c) of spores, which corresponds to 50% reduction in viability, was decreased from 405 degrees C when heated at similar to 10(4) degrees C/s in air to 250 degrees C when heated at the same rate in 100 ppm Cl-2. SEM results show no obvious difference between the morphologies of spores heated in air or in Cl-2 at similar to 10(4) degrees C/s. These results indicate that Cl-2 gas acts in synergy with high temperatures ( > 300 degrees C) to neutralize Bt spores. Similarly, the T-c for Bt spores heated at the faster rate of similar to 10(5) degrees C/s was reduced from 230 degrees C when heated in air to 175 degrees C when heated in 100 ppm Cl-2. At similar to 105 degrees C/s, the treatment of Cl-2 did not alter spore morphology at temperatures below 300 degrees C. At temperatures above 450 degrees C with Cl-2, the spore coat detached from the underlying core. The effect of Cl-2 was further examined by changing the RH of Cl-2 gas. The results show that highly humidified Cl-2 (RH=100%) reduced Tc by 170 degrees C and 70 degrees C at similar to 10(4) degrees C/s and similar to 10(5) degrees C/s, respectively, as compared to dry Cl-2 (RH=0%). Energy dispersive spectrometric (EDS) results demonstrate that Cl-2 on the spore increased with elevated peak temperature, with the majority of the Cl located in the shed spore coat. This study indicates that the major mechanism of spore neutralization by the synergism of Cl-2 and rapid heat is chlorine reacting with the spore surface. (C) 2016 Published by Elsevier Ltd.