Abstract Crosslinking resulting from thermo and photo-oxidation of ethylene–propylene–diene-monomer (EPDM) and polycyclo-octene (Vestanemer) has been studied by refractometry and X-ray diffraction. Refractometry consists of the measurement of the changes in the refractive index of the soluble part of elastomer that are caused by oxidation: it is shown that the refractive index tends to that of the solvent when crosslinking of the elastomer occurs as a result of oxidation. X-ray diffraction permits measuring the variation of crystallinity during oxidation. It is observed that crosslinking decreases the ability of the macromolecular chains to crystallise. The behaviours of both the elastomers in conditions of thermal and photo-oxidation have been compared.