This part of ISO 13164 specifies a test method for the determination of radon-222 activity concentration in a sample of water following the measurement of its short-lived decay products by direct gammaspectrometry of the water sample (see Annex A). The radon-222 activity concentrations, which can be measured by this test method utilizing currently available gamma-ray instruments, range from a few becquerels per litre to several hundred thousand becquerels per litre for a 1 l test sample. This test method can be used successfully with drinking water samples. The laboratory is responsible for ensuring the validity of this test method for water samples of untested matrices. Annex B gives indication on the necessary counting conditions to meet the required sensitivity for drinking water monitoring.