Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of American Chemical Society for personal use, not for redistribution. The definitive version was published in Environmental Science & Technology 45 (2011):1298–1306, doi:10.1021/es103838p. ; Response actions to the Deepwater Horizon oil spill included the injection of ~771,000 gallons (2,900,000 L) of chemical dispersant into the flow of oil near the seafloor. Prior to this incident, no deepwater applications of dispersant had been conducted and thus no data exists on the environmental fate of dispersants in deepwater. We used ultrahigh resolution mass spectrometry and liquid chromatography with tandem mass spectrometry (LC/MS/MS) to identify and quantify one key ingredient of the dispersant, the anionic surfactant DOSS (dioctyl sodium sulfosuccinate), in the Gulf of Mexico deepwater during active flow and again after flow had ceased. Here we show that DOSS was sequestered in deepwater hydrocarbon plumes at 1000-1200m water depth and did not intermingle with surface dispersant applications. Further, its concentration distribution was consistent with conservative transport and dilution at depth and it persisted up to 300 km from the well, 64 days after deepwater dispersant applications ceased. We conclude that DOSS was selectively associated with the oil and gas phases in the deepwater plume, yet underwent negligible, or slow, rates of biodegradation in the affected waters. These results provide important constraints on accurate modeling of the deepwater plume and critical geochemical contexts for future toxicological studies. ; The authors gratefully acknowledge funding from the National Science Foundation’s RAPID program (OCE-1045811 to EBK, OCE-1042097 to DLV, OCE-1042650 to J. D. Kessler for R/V Cape Hatteras cruise) and from the WHOI Director of Research. Instrumentation in the WHOI FT-MS facility was funded by the National Science Foundation MRI program (OCE-0619608) and by the Gordon and Betty T. Moore Foundation. Stipend support for A. Boysen was provided by the WHOI Summer Student Fellow Program.