In practice, sealing materials are always used under deformation and are subject to various aging processes as a function of ambient factors for example, temperature and oxygen, frequently manifested in stress relaxation impacted critically with respect to the long-term functioning of the seal. This study thus aims to contribute to a detailed understanding of the specific aging mechanism of stress relaxation in the case of EPDM elastomers. To this end, the influence of material and ambient parameters on irreversible changes in the crosslinking and chemical structures of the polymer matrix under mechanical load was extensively examined. Attainment of the targeted objective of this study involved further development and optimization of innovative characterization methods like low-field time-domain NMR spectroscopy with inverse Laplace transformation (ILT) and chemiluminescence under deformation. The NMR methods and the ILT analysis methods further developed here were used to describe as precisely as possible the structural changes occurring in the network and with respect to chain mobility. With the help of this method, it was possible to quantify structural changes like crosslinking, the share of free chain ends and free chain fragments formed during the aging process. All the aforementioned material parameters were found to have a significant impact on the aging-related change in stress relaxation at constant deformation. The rate of thermal oxidation under deformation was also shown to be greater than in a non-deformed state. The new characterization methods allow for a much more precise description, and thus a markedly better understanding, of the stress relaxation mechanism among elastomers.