This part of ISO 13164 specifies a test method for the determination of radon-222 activity concentration in a sample of water following its transfer from the aqueous phase to the air phase by degassing and its detection. It gives recommendations for rapid measurements performed within less than 1 h. The radon-222 activity concentrations, which can be measured by this test method utilizing currently available instruments, range from 0,1 Bq l"1 to several hundred thousand becquerels per litre for a 100 ml test sample. This test method is used successfully with drinking water samples. The laboratory is responsible for ensuring the validity of this test method for water samples of untested matrices. This test method can be applied on field sites or in the laboratory. Annexes A and B give indications on the necessary counting conditions to meet the required sensitivity for drinking water monitoring.