Water primrose (Ludwigia hexapetala) is an invasive aquatic vegetation that has rapidly increased in coverage throughout the Sacramento-San Joaquin Delta. Water primrose has invaded wetlands and may contribute towards mortality in tule (Schoenoplectus spp.) and cattail (Typha spp.). Little research has been completed on the mechanisms responsible for marsh loss, but previous studies have suggested that water primrose contains allelopathic chemicals that could cause plant mortality. Three major allelopathic polyphenols (myricitrin, prunin, and quercitrin) previously identified in L. hexapetala leaves were measured in leaf, water, and soil samples from Delta marshes infested with water primrose. Marshes analyzed in summer, fall, and spring included Big Break, Latham Slough, Liberty Island, and Sherman Lake. Rhode Island and Venice Cut were analyzed in Summer. Samples were collected at the interface between water primrose and the marsh and within the patch of floating water primrose. In analyzing leaf samples by liquid chromatography tandem mass spectrometry, ion ratios and retention times of the prunin reference standard did not align with the analyte in the samples (i.e., prunin was not detected water primrose from the Delta). Further analysis by high resolution mass spectrometry and library matching resulted in the tentative identification of salipurposid, a structural isomer of prunin, as the potential third major polyphenol expressed by water primrose in the Delta. For the purpose of this study, salipurposid was semi-quantitated using the prunin reference standard to give concentration values. In leaves, concentrations of myricitrin, salipurposid, and quercitrin ranged 900-4397 µg/g dry weight (d.w.), 7-2913 µg/g d.w., and 363-3956 µg/g d.w., respectively. Analysis of water samples for polyphenols found the highest number of detections in summer, with many non-detects in fall and spring. Polyphenols were detected in soil samples throughout the year. Quercitrin was the most frequently detected polyphenol in water (53%, 5-3660 ng/L) and soil (67%, 11-1766 ng/g d.w.). The proliferation of water primrose likely threatens the Delta ecosystem. Additional studies could help improve understanding of how water primrose affects native marsh habitat and causes mortality of emergent macrophytes.