Carbonization, oxidation and hydrogenation in expanded polystyrene (EPS) resulted from its thermal degradation at 120 A degrees C are studied in this work using X-ray photoelectron spectroscopy. This hardly biodegradable polymer accumulates large quantities of solid wastes because it is commonly used in disposable dishes and containers. The objective of the work was to obtain a quantitative measure of its thermal degradation at four different depths from the surface, 0, 30, 60 and 90 nm based on the evolution of its chemical states as a function of the heating time. At least ten carbon chemical states were identified, five belonged to the EPS structure, and the others appeared due to the thermal degradation in the form of multiple chemical bonds. The results indicated that carbonization and dehydrogenation were the main degradation mechanisms of the thermal treatment. During the first 7 h of heating, carbonization increased 6.6%, hydrogenation decreased 6.2% and oxidation decreased 2.5%. The surface had more oxidation and behaved differently from the interior of the material. As most atoms in EPS are C, it was considered that the difference in carbonization percentages could represent the degradation percentage of the material.