This document specifies a method for the measurement of 14C activity concentration in all types of water samples by liquid scintillation counting either directly on the test sample or following a chemical separation. The method is applicable to test samples of supply/drinking water, rainwater, surface and ground water, marine water, as well as cooling water, industrial water, domestic, and industrial wastewater. The detection limit depends on the sample volume, the instrument used, the sample counting time, the background count rate, the detection efficiency and the chemical recovery. The method described in this document, using currently available liquid scintillation counters and suitable technical conditions has a detection limit as low as 1 Bq l"1, which is lower than the WHO criteria for safe consumption of drinking water (100 Bq·l-1). 14C activity concentrations can be measured up to 106 Bq l-1 without any sample dilution. It is the user s responsibility to ensure the validity of this test method for the water samples tested.