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The relationship between chemical changes and changes of molecular properties of elastomeric materials through thermo-oxidation has been evaluated. Scission and recombination reactions compete during oxidative ageing. In the case of natural rubber, the molecular evolution exhibits a dual process, whereas, rapid crosslinking occurs from the beginning of the thermo-oxidation of EPDM. A chemical change has been simultaneously detected by FTIR for the two kinds of diene materials. These specific behaviours are compared to those upon photo-oxidation. No difference is recorded for NR. In contrast, the chronology of events depends on the process of oxidative ageing in case of EPDM.