This document specifies a method for the physical pre-treatment and conditioning of water samples and the determination of the activity concentration of various radionuclides emitting gamma rays with energies 40 keV < E < 2 MeV, by gamma ray spectrometry according to the generic test method described in ISO 20042[9]. NOTE The determination of the activity concentration of radionuclides emitting gamma rays with energy below 40 keV and above 2 MeV is also possible within the scope of this document, provided both the calibration of the measuring system and the shielding are adapted to this purpose. This document is only applicable to homogeneous samples. The lowest limit that can be measured as such, i.e. without dilution or concentration of the sample or anti Compton device is about 5.10-2 Bq/l for eg 137Cs. The upper limit of the activity corresponds to a dead time of 5%. Depending on different factors, such as the energy of the gamma rays and the emission probability per nuclear disintegration, the size and geometry of the sample and the detector, the shielding, the counting time and other experimental parameters, the sample is concentrated by evaporation when activities below 5.10-2 Bq/l have to be measured. However, volatile radionuclides (e.g. radon and radioiodine) can be lost during the source preparation. When the dead time is higher than 5%, the sample is either diluted or an aliquot of the sample is taken or the source to detector distance is increased or a correction for pile up effects is applied.