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The historical occurrence of Dechlorane Plus (DP) and detection of novel compounds structurally related to DP is described in a dated Lake Ontario sediment core. Our core was collected near the mouth of the Niagara River, which is known to be a major source of DP to the lake. Maximum DP concentrations (920 ng g(-1), dry weight) were observed between 1976 and 1980, the highest reported to date. Following that time, we observed a dramatic decrease in DP concentration which coincided with the enactment of United States federal and state laws to mitigate free release of chemicals into the Niagara River and installation of an industrial wastewater treatment facility. During the course of our research, four new substances structurally related to DP were also identified. These compounds were thought to arise from the Diels-Alder reactions resulting from impurities present in 1,5-cyclooctadiene, a feedstock used in production of DP. To confirm our hypothesis, Diels-Alder reactions were performed on the individual impurities. Using different stationary-phase capillary gas chromatography columns and high-resolution mass spectrometry, we were able to positively identify some of these novel compounds in the core. Interestingly, we also were able to identify a monoadduct compound, formed by addition of 1 mol of hexachlorocyclopentadiene to 2 mol of 1,3-cyclooctadiene, in lake trout. The concentration of this monoadduct was approximately 2 orders of magnitude greater than that of DP, suggesting that it is more bioaccumulative.