The bromine content from high impact polystyrene containing decabromo diphenyl oxide flame retardant (HIPS-Br) was removed by hydrothermal treatment using an autoclave. The debromination process was promoted by the addition of PP to HIPS-Br. A temperature of 280 °C was found to be optimum for a high yield of recovered plastic with maximum debromination of HIPS-Br. The recovered plastic can be used as a substitute for liquid fuel, which is the product during feedstock recycling. The process is very simple, but more attractive than feedstock recycling as this process eliminates the costly and complicated catalytic process for the removal of halogen from the waste plastic derived oil.