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Even though high density polyethylene is one of the most stable polymers, additives must still be placed in geosynthetic products to ensure long term stability. The types of additives used include phosphites for process stabilization, hindered phenols for long term oxidative stability and carbon black or hindered amines for protection from UV radiation. The effectiveness of additives is often determined by exposure to elevated temperatures in a forced-air oven. The purpose of this study was to evaluate the effect of oven exposure on HDPE geomembranes made with different stabilizers. Samples of geomembranes made with fifteen different additive packages were aged for 500 days at 115 • C and some of their properties were monitored. The results showed that the melt flow rate is a test sensitive to the effects of air-oven aging. Additionally, it was shown that hindered amines were the best long term stabilizers and that the addition of carbon black actually reduced the oxidative stability of the products.