We used 222Rn and 224Ra measured in the Upper Gulf of Thailand to estimate 222Rn atmospheric evasion across the water-air interface. The Chao Phraya estuary represents a steady-state source of these tracers and we looked at their horizontal distribution on a transect leading away from the estuary into the Gulf. The isotopes 222Rn and 224Ra have very similar half-lives and are affected in the same manner by mixing processes but only radon will emanate to the atmosphere. We thus are able to estimate the radon air-water exchange rate from the difference in the slopes of the 222Rn and 224Ra horizontal distributions. Estimated gas exchange velocities (k) based on our results were 1.1 cm hr-1 in the dry season (January 2004) and 2.1 cm hr-1 during the wet season (July 2004). These rates agreed reasonably well with some theoretical models developed for lakes, estuaries and coastal systems.