Sea surface radon-222 measurements were carried out onboard the research boat Gridi. The survey was run at 1.4 knots, with the boat position recorded by DGPS. Two pumps that continuously supplied water for the measurements were fixed at ca. 50 cm water depth with a Sea & Sun Technology CTD90M multiprobe that was recording pressure (depth), temperature and conductivity (salinity). The radon measurements were carried out using two identical systems in parallel, each equipped with a 3M MiniModule gas contractor that separates the dissolved gas from the continuously pumped water. The gas was dried with a Drierite gas-drying unit, and analysed with a Durridge RAD7 radon detector. The radon-in-water activity concentrations were calculated using the salinity and temperature dependent fractionation of radon-222 between air and water.