This document specifies conditions for the determination of 90Sr and 89Sr activity concentration in samples of environmental water using liquid scintillation counting (LSC) or proportional counting (PC). The method is applicable to test samples of drinking water, rainwater, surface and ground water, marine water, as well as cooling water, industrial water, domestic, and industrial wastewater after proper sampling and handling, and test sample preparation. Filtration of the test sample and a chemical separation are required to separate and purify strontium from a test portion of the sample. The detection limit depends on the sample volume, the instrument used, the sample count time, the background count rate, the detection efficiency and the chemical yield. The method described in this document, using currently available LSC counters, has a detection limit of approximately 10 mBq l−1 and 2 mBq l−1 for 89Sr and 90Sr, respectively, which is lower than the WHO criteria for safe consumption of drinking water (100 Bq·l−1 for 89Sr and 10 Bq·l−1 for 90Sr). These values can be achieved with a counting time of 1 000 min for a sample volume of 2 l. The methods described in this document are applicable in the event of an emergency situation. When fallout occurs following a nuclear accident, the contribution of 89Sr to the total amount of radioactive strontium is not negligible. This document provides test methods to determine the activity concentration of 90Sr in presence of 89Sr. The analysis of 90Sr and 89Sr adsorbed to suspended matter is not covered by this method. It is the user’s responsibility to ensure the validity of this test method selected for the water samples tested.