AbstractTwo sparging devices have been designed, built, and tested. They can be used to estimate the concentration of truly dissolved chemical in water and thus the potential for diffusive transfer by volatilization. Samples of air, and air contacted to achieve equilibrium with the water, are analyzed to give an unequivocal determination of the displacement of equilibrium between air and water. Essentially, the chemical's fugacity or partial pressure is measured. A theoretical treatment is presented. Exploratory test results in a wind‐wave tank and in the field are described and discussed. It is believed that these devices are of value for assessing the sorbed vs. dissolved condition of hydrophobic chemicals in natural waters that influences not only volatilization but also other diffusive processes such as bioconcentratio