Melting and chemical behaviors of isothermally crystallized gamma-irradiated sPS have been investigated using differential scanning calorimetry, FTIR, and X-ray diffraction techniques. Amorphous sPS samples were subjected to gamma radiation in vacuum and in oxygen at different doses from 200 to 1000 kGy. Irradiated samples were heated to 310 degrees C, cooled to 220-260 degrees C range, held for 10 min, and re-melted. Three melting endothermic peaks observed for irradiated and non-irradiated samples isothermally crystallized at 220 degrees C were decomposed into individual Gaussian distributions, and enthalpies of the total melting endotherm and individual peaks were determined. Both alpha and beta crystalline forms coexist in the crystallized irradiated sPS, regardless of the radiation treatment environment. Dose and irradiation environment have a great effect on the melting behaviors and chemical structures of the isothermally crystallized gamma-irradiated sPS. Crystallinity increases with increasing dose of irradiation in both vacuum and oxygen, with level of increase is greater in oxygen. The three melting peak temperatures decrease with increasing dose regardless of irradiation environments. Ketone and aldehyde oxidized products are formed in the isothermally crystallized gamma-irradiated sPS in oxygen through a combination of hydrogen abstraction or chain scission process and hydroxyl free radicals formation. (C) 2008 Elsevier Ltd. All rights reserved.