The first detailed evidence for dynamic air−water exchange of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) is presented. Samples of air (340−380 m3) and water (33−60 L) were taken simultaneously during July 1998 at two sites in the lower Hudson River Estuary, NY. The atmospheric gas and particulate phases and the aqueous dissolved and particulate phases were analyzed for di- to octa-CDD/Fs. All the homologue groups were routinely detected by HRGC-HRMS, with detection limits for the homologue groups 1 pg/sample. Cl2DDs, OCDD, and Cl2DFs were the most abundant homologues in the water, and the Cl2DDs were the most abundant in the air (4.3−7.6 pg/m3). The Cl2DD/Fs and Cl7/8DD/Fs were 25−53% and 78−99% associated with the water particulate phase, respectively. The likelihood of sampling artifacts influencing the apparent dissolved/particulate partitioning of the higher chlorinated congeners is discussed. Water concentrations were constant over the sampling period, while atmospheric concentrations varied with air mass origin. The fugacity ratios between the dissolved phase in water and the gas phase in air were usually >1, implying a net volatilization flux. Evidence for outgassing of the lower chlorinated homologues, obtained by the simultaneous measurement of air over adjacent land and water, provided further support for the outgassing of the lower chlorinated homologues from the water body.