Accelerated ageing tests of polychloroprene rubber O-rings were carried out to evaluate the long-term reliability of the sealing performance of O-rings used in gas-insulated substation equipment, such as transformers and switchgear. The results of the tests confirmed the strong influence of atmospheric oxygen on the lifetime of the rubber O-rings, as well as the influence of temperature. Several physical and chemical changes were found to proceed in the degradation of the rubber O-rings, in particular stress relaxation due to plastic deformation, decline of viscoelasticity due to the formation of new cross-links by residual vulcanizing agent or oxidation by atmospheric oxygen, and bond scission in the rubber molecule due also to oxidation by atmospheric oxygen. The rate of the degradation of the rubber O-rings varied depending on the ambient conditions. Finally, a degradation model of polychloroprene O-rings was proposed, and empirical formulae for some conditions were provided based on approximation analyses using the accelerated ageing tests.