The kinetics of the thermal-oxidative decomposition of expandable polystyrene (EPS) was studied by a conventional thermogravimetric technique in various concentrations of oxygen from 0 to 21%. A kinetic model that accounts for the effects of oxygen concentration was proposed to describe the thermal-oxidative decomposition of EPS. The thermogravimetric analysis curve and its derivative have been analyzed by using differential and integral methods with modification of the Friedman and Coats-Redfern methods. The activation energy, the pre-exponential factor, and the reaction order for unreacted material and oxygen concentration have been determined. When oxygen was present, the activation energy was reduced significantly.