This article deals with the Swift Heavy Ion (SHI) irradiation of atactic polystyrene at different doses, up to about 10 MGy, under an inert as well as under an oxidative atmosphere. The effect of the irradiation is adressed using various analytical tools such as high-resolution gas mass spectrometry, gel fraction, size exclusion chromatography, and Fourier Transform InfraRed spectroscopy. It has been evidenced that under SHI, in conditions of homogenous oxidation, part of the defects identified are those commonly observed under an oxidative atmosphere with low linear energy transfer irradiations (hydroperoxides and carbonyl bonds for instance) and part of the observed defects are those generally observed under inert atmosphere (C=C bonds...). This specific effect is assigned to the heterogeneous structure of energy deposition with ion beams. More precisely, this is due to the high ionization and excitation density in ion tracks. PS is known to be very radiation-resistant under inert atmosphere. Althought degradation is increased under oxidative atmosphere, the oxidation remains very low compared to the one observed in polyethylene; even at doses as high as 10MGy. This shows the efficiency of the radiation protection effect of the aromatic rings under oxidative atmosphere.