Ethylene Norbornene Copolymers (ENCs) belong to the Cyclic Olefin Copolymers (COC) family and are used in numerous industrial fields. The goal of this paper is to compare the behavior under electron beam radiation of three grades of ENCs. Several radiation doses have been tested: 25 kGy to simulate sterilization dose and higher doses (75 and 150 kGy) to emphasize the electron beam effect on polymer ageing. By using Triple Detection (Refractometer/Laser Scattering/Viscometer) Size-Exclusion Chromatography, we wanted to study more accurately the evolution of ENCs under radiation than the previous studies available. Thanks to Triple detection, molecular weights were calculated without being dependent of calibration standards, branching was evidenced as the major effect of electron beams on those materials over a dose of 25 kGy, and long chain branching and branching chains frequencies were also calculated. This study focuses on the effect of additive concentration and norbornene (NB) content and a trend has merged: for a same NB content, the higher the phenolic antioxidant concentration, the less crosslinked was the copolymer with the dose. By increasing NB content, radiation effects were minimized.