Ab initio calculations by using Gaussian-2 theory have been carried out for the reactions between halogen atoms and various fuel molecules, i.e. fluorine, chlorine, and bromine atoms vs. hydrogen, methane, ethane, ethylene, acetylene, ammonia, silane, dichlorosilane and phosphine. The activation energy for the reaction between a halogen atom and a fuel molecule seems to indicate whether the reaction between the fuel gas and the corresponding halogen gas occurs spontaneously when they are brought into contact to each other at room temperature.