This International Standard specifies the conditions for the determination of tritium activity concentration in samples of marine, surface, ground, rain drinking waters or of tritiated water ([3H]H2O) in effluents using liquid scintillation counting. The choice of the analytical procedure (either with or without distillation of the water sample prior to determination), depends on the aim of the measurement and the sample characteristics[20],[21],[22]. Direct measurement of a raw water sample using liquid scintillation counting[9] has to consider the potential presence of other beta emitter radionuclides. To avoid interference with these radionuclides when they are detected, the quantification of tritium is performed following the sample treatment by distillation.[23],[24],[25],[26] The Annexes B, D and E describe three distillation procedures. The method is not directly applicable to the analysis of organically bound tritium; its determination requires additional chemical processing of the sample (such as chemical oxidation or combustion). With suitable technical conditions, the detection limit may be as low as 1 Bq l"1. Tritium activity concentrations below 106 Bq l"1 can be determined without any sample dilution. A prior enrichment step can significantly lower the limit of detection[27],[28].