This document specifies a method by liquid scintillation counting for the determination of tritium activity concentration in samples of marine waters@ surface waters@ ground waters@ rain waters@ drinking waters or of tritiated water ([3H]H2O) in effluents. 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.