This International Standard specifies the conditions for the determination of 14C activity concentration in samples of environmental water or of 14C-containing water using liquid scintillation counting. The method is applicable to the analysis of any organic molecule soluble in water that is well mixed with the scintillation cocktail. It does not apply to micelles or “large” particles (lipids, fulvic acid, humic acid, etc.) that are inadequately mixed with the scintillation cocktail and the water. Some beta energy is lost without any excitation of the scintillation cocktail and the results are underestimated. The method is not applicable to the analysis of organically bound 14C, whose determination requires additional chemical processing (such as chemical oxidation, combustion). It is possible to determine 14C activity concentrations below 106 Bq l−1 without any sample dilution.