Because of its special properties and commercial significance, Poly(phenylene sulfide) (PPS) has been the subject of many research efforts since its commercial introduction in 1967. Intensive work has been done on its crystalline structure and morphology and its thermal behaviour. But fewer investigations have been carried out to understand long term behaviour in high temperature environments. Always anticipating industrial needs linked to power integration, we have launched an extensive study on thermal aging in air of PPS at 250°C. This study has shown that PPS thermal degradation in air happens by intermolecular branching reaction, similar to crosslinking. This phenomenon was already known for temperature above 300°C. This crosslinking is evidenced by rheometry where the relative position of G’ and G’’ above melting temperature changes with aging. IR spectroscopy confirms that para substituted benzene in PPS molecule is transformed in 1,2,4 trisubstituted benzene. DSC measurements evidence both an elevation of melting temperature and a change in melting endotherm showing significant changes in crystalline morphology along aging, which tends to indicate that crosslinking occurs in crystalline phase. Then degradation implies drastic loss of mechanical properties leading to destruction of the sample.