The invention relates to a method and a device for continually and especially quickly detecting changes in the concentration of radon gas by means of conversion into a measuring gas (Rn-222) which can be used for varied monitoring, controlling and regulating tasks, whereby said gas is dissolved in water. A membrane is permeable to the radioactive noble gas radon but is essentially impermeable to water. The invention is based upon said membrane being flown round by the radon-containing water on the one side and by a carrier gas on the other side in parallel or in opposite directions and involving flow rates which are optimised respectively. The concentration of radon in the measuring gas is directly proportional to the concentration of radon in water when stable marginal conditions are guaranteed.