Triclosan (TCS) is a commonly used antimicrobial agent which persists in the environment and may undergo chlorination and/or photodegradation to produce toxic polychlorinated dibenzo-p-dioxins and polychlorinated benzenes. TCS accumulates in wastewater treatment biosolids, which may be used to fuel waste-to-energy plants, although little is known about the fate of TCS at high temperatures. Here, we have studied the thermal decomposition of TCS and chlorinated TCS derivatives in the gas phase using computational chemistry coupled with reaction rate theory calculations to predict rate coefficients and develop a chemical kinetic model to simulate TCS pyrolysis in a plug flow reactor. TCS is shown to interconvert with 4-chloro-2-(2,4-dichlorophenoxy)phenol (TCSi) with a relatively low barrier, achieving equilibrium at temperatures of around 900 K and above. Dissociation of TCS and TCSi proceeds in parallel with barriers of ca. 60-65 kcal/mol to produce dichlorodibenzo-p-dioxin chlorobenzoquinone isomers. Reactor simulations demonstrate that TCS incineration at a temperature of 1100 K or higher leads to the formation of toxic chlorinated aromatics.