This part of ISO 18589 describes the principles for the measurement of the activity of 90Sr in equilibrium with 90Y, and 89Sr, pure beta-emitting radionuclides, in soil samples. Different chemical separation methods are presented to produce strontium and yttrium sources, the activity of which is determined using proportional counter (PC) or liquid scintillation counter (LSC). The selection of the measuring method depends on the origin of the contamination, the characteristics of the soil being analysed, the required accuracy of measurement and the resources of the available laboratories. These methods are used for soil monitoring following past or present, accidental or routine, liquid or gaseous discharges. It also covers the monitoring of contamination caused by global fallout. In the case of recent fallout immediately following a nuclear accident, the contribution of 89Sr to the total amount of strontium activity is not negligible. This part of ISO 18589 provides the measurement method to determine the activity of 90Sr in the presence of 89Sr.