This document specifies a test method for the determination of iron-55 (55Fe) activity concentration in samples of all types of water using liquid scintillation counting (LSC). Using currently available liquid scintillation counters, this test method can measure the 55Fe activity concentrations in the range of less than 30 mBq l-1 to 120 mBql-1. These values can be achieved with a counting time between 7 200 s and 10 800 s for a sample volume from 0,5 l to 1,5 l. Higher activity concentrations can be measured by either diluting the sample or using smaller sample aliquots or both. NOTE These are indicative values. Individual laboratories should establish their own performance parameters. The range of application depends on the amount of dissolved material in the water and on the performance characteristics of the measurement equipment (background count rate and counting efficiency). It is the laboratory s responsibility to ensure the suitability of this test method for the water samples tested.