Numerous studies have been performed in the past to analyse the ability of barrier treatment technologies to control permeation of gasoline and gasohol blends through HDPE fuel tanks. These results have been both inconclusive and contradictory. A research program was recently implemented to determine the source of these discrepancies and to supply definitive performance data. One conclusion from this study is that processing parameters play a critical role in barrier effectiveness. Control of critical initial parameters significantly improves the ability of barrier treatment technologies, such as fluorination, to control hydrocarbon emission losses through HDPE fuel tanks. Conventional permeation tests reveal some unexpected relationships.