The deterioration of polyethylene and polyvinyl chloride cable materials in the containment building of an operating nuclear reactor has been investigated. Since the maximum dose experienced by the cable materials was only 2.5 Mrad during ∼12 yr of operating life, the extent of material degradation was surprising. Laboratory aging experiments on the two materials established that the cause of the material deterioration in the plant was radiation-induced oxidation. The degradation rate was correlated with local levels of radiation intensity. It was determined that strong synergisms of radiation and elevated temperature, and also dose-rate effects, lead to the surprisingly rapid degradation rates found with these materials. It is concluded that in the design of laboratory methods for aging and qualification testing of organic materials for use in a nuclear plant environment, the possible occurrence of dose-rate effects and synergisms needs to be taken into account.