Bioaccumulation of methylmercury in coastal U.S. fisheries has led to the issuance of numerous fish consumption advisories, and yet little is known about the processes that make Hg species chemically labile in coastal and estuarine systems. This study examined the role of estuarine mixing in formation of labile Hg complexes (reactive Hg) from relatively refractory Hg-organic associations in river water and characterized the behavior and distribution of Hg species in the Connecticut River estuary during three distinct collection periods. Results indicate that while total Hg partitioning and concentrations remained fairly constant with increasing salinity, the fraction present as reactive Hg concentrations increased, primarily in the particulate phase. Mixing experiments using both natural and prepared waters indicate that riverine organic ligands rapidly scavenge reactive Hg from natural waters on timescales of minutes to hours, while samples free of riverine influence remained much more reactive. Modeling of the estuarine system suggests that elevated concentrations of chloride and dilution of the dominant organic ligand associated with estuarine mixing enhance reactive Hg and predict a bulk log formation constant for the binding ligand of approximately 21. Analysis of Hg0 production from Hg(II)-spiked, incubated estuarine samples supports the speciation data as higher reactive Hg concentrations and Hg0 production rates were observed in the more saline samples. These results suggest that estuarine mixing may exacerbate Hg methylation, evasion, and bioaccumulation in some systems by promoting the formation of Hg species that are readily labile.