This International Standard specifies the test methods and their associated principles for the measurement of the activity of 90Sr in equilibrium with 90Y, and 89Sr, pure beta-emitting radionuclides, in water samples. Different chemical separation methods are presented to produce strontium and yttrium sources, the activity of which is determined using a proportional counter (PC) or liquid scintillation counter (LSC). The selection of the test method depends on the origin of the contamination, the characteristics of the water to be analysed, the required accuracy of test results and the available resources of the laboratories. These test methods are used for water monitoring following, past or present, accidental or routine, liquid or gaseous discharges. It also covers the monitoring of contamination caused by global fallout. When fallout occurs immediately following a nuclear accident, the contribution of 89Sr to the total amount of strontium activity is not negligible. This International Standard provides the test methods to determine the activity concentration of 90Sr in presence of 89Sr.