A new method of quantifying mean rates of gas exchange for natural water systems using the distribution of pollutant chlorofluorocarbons (CFCs) is presented. Concentrations of dissolved CCl3F (F-11), CCl2F2 (F-12), and CCl2FCClF2 (F-113) in the Hudson estuary are supersaturated with respect to the remote troposphere by as much as an order of magnitude. The most plausible source appears to be discharges from waste water treatment facilities. Loss of these compounds from the estuary water to the atmosphere occurs both upstream and downstream of the zone of input. Using a multi-box model, gas exchange coefficients were calculated to be between 2 and 4 cm hr-1 based on the observed concentrations. This rate of gas exchange is similar to values determined in lakes and coastal bays and substantially lower than the mean value for the open ocean. ¿ American Geophysical Union 1992