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Physical processes such as advection, dispersion, and air-water gas exchange play important roles in determining the movement and change in concentration of contaminants discharged into rivers. In the following, we report results from a large-scale SF6 tracer release experiment conducted in the tidal Hudson Riverto examine longitudinal dispersion and net advection. SF6 was injected into the Hudson River near Newburgh, NY, and surveyed for 13 days using a new, fully automated, high-resolution SF6 sampling and analysis system. Net down river advection of the water body originally tagged with SF6 was slow, averaging mean displacement rates of about 0.5 +/- 0.2 km d(-1). In contrast, spreading of the tracer was driven by tidal movement, causing rapid mixing of the water up and down river. By examining the change in the second moment of the tracer distribution with time, we determined the mean longitudinal dispersion coefficient to be 70.1 +/- 4.3 m2 s(-1). Temporal evolution of the SF6 inventory indicates an average gas transfer velocity over the period of the experiment of 6.5 +/- 0.5 cm h(-1) (1.56 +/- 0.12 m d(-1)). Vertical profiles show that mixing into the bottom layers of the river, in places reaching more than 53 m, seemed to be rapid.