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.