Gas exchange across the air-water interface is a key parameter when using gas tracers such as radon to quantify groundwater discharge to surface waters. We present a novel method for quantifying the gas transfer velocity based on recently developed techniques for the in situ, near continuous measurement of dissolved gases with a field portable mass spectrometer. Concentrations of gases in surface water show diurnal variations due to diurnal changes in water temperature (and thus gas solubility). However, variation in observed dissolved gas concentrations are damped and lagged with respect to equilibrium concentrations, the extent of which depends upon the diurnal temperature variation, the water depth, and the gas transfer velocity. The method fits a model to the measured gas concentrations to derive the gas transfer velocity from the amplitude and the phase lag between observed and equilibrium concentrations. With the current experimental setup, the method is sensitive to gas transfer velocities of 0.05 – 9 m/day (for N2), at a water depth of 1 m, and a given daily water temperature variation of 10 ºC. Experiments were performed (a) in a controlled experiment to prove the concept and to confirm the capability to determine low transfer velocities, and (b) in a field study in a shallow coastal lagoon covering a range of transfer velocities.