A robust and straightforward method for on-site determination of radon-222 activity concentrations in water is presented. The methodical approach is based on the principle of liquid-gas-membrane extraction. An extraction module, which consists of hollow polypropylene fibres, allows radon stripping from the water of interest into a connected closed air-loop. The resulting radon-in-air activity concentration, which can easily be reconverted into the original radon-in-water activity concentration is measured by means of a standard mobile radon-in-air monitor. The experimental set-up allows radon detection in discrete water samples as well as continuous water pump streams. The technique, covering a wide activity concentration range, is in particular advantageous for measurements of radon-in-water in the field or on research cruises. Crown Copyright © 2008.