The supercritical fluid extraction (SFE) of twenty one polychlorinated biphenyls (PCBs) and eight organochlorine pesticides (OCPs) from freeze dried mussel tissue is discussed, demonstrated, and compared to traditional Soxhlet extractions. The aim of this study was to determine the efficacy of SFE for the extraction of these lipophilic compounds which involved a spike study to determine the feasibility of on-line sample clean-up by an inclusion of activated alumina within the extraction vessel. These spike extractions from an inert matrix showed that the chemical integrity of the PCBs was not compromised while some OCPs of the DDT derivative were decomposed to other forms. Preliminary extractions of freeze dried mussel tissue focused on the effect of static extraction time and CO₂ density on recovery of twenty three specific congeners of PCBs. These extractions of freeze dried mussel tissue showed longer static extraction time and higher density/lower temperature would yield quantitative recoveries of PCBs. Triplicate extractions of mussel tissue at the appropriate extraction parameters produced quantitative and similar recoveries to Soxhlet. Finally, an alternative supercritical fluid (CHF₃) yielded non-quantitative recoveries for PCBs and OCPs, but the use of this fluid eliminated the need to use in situ alumina to retain co-extracted lipids from the matrix. / Master of Science