Plastics are widely used and they vary in their applicability, ranging from automobile parts, components for houses and buildings, and packaging for everything from food to electronic parts. The diverse applications of plastics, such as polystyrene, polyolefins and polyester, are credited to the incorporation of additives. Additives improve the performance of these and other polymer resins. Without the incorporation of such additives, for example Ethanox ® 330, some plastics would degrade during processing or over time. To ensure that the specified amount of an additive or combination of additives are incorporated into a polymer after the extrusion process, a rapid and accurate analytical method is required. Quantitation of additive(s) in the polymer is necessary, since the additive(s) may degrade and the amount of additive(s) can influence the physical nature of the polymer. Conventional extraction techniques for polymer additive(s), such as, Soxhlet or dissolution / precipitation are labor intensive, time consuming, expensive, and the optimal recovery is significantly less than 90 percent. In addition, a large amount of solvent , such as toluene or decalin, must be eliminated in order to concentrate the sample prior to chromatographic separation. Supercritical Fluid Extraction (SFE) has been employed as an alternative polymer preparation technique. SFE is a favorable means for various analytical sample preparation applications, credited to its short extraction times. This research employs SFE for the extraction of the antioxidant Ethanox® 330 from high density polyethylene (HDPE) followed by HPLC/UV analysis. The effects of temperature, modifier type, and modifier concentration were investigated. Once the optimal extraction conditions were determined, the extraction efficiency of Ethanox ® 330 as a single additive and in the presence of co-additives from HDPE were investigated. Recoveries of greater than 90% were obtained for Ethanox ® 330 when a secondary antioxidant was present in the HDPE. / Master of Science