This document specifies a method for the determination of the dissolved fraction of selected active pharmaceutical ingredients and transformation products, as well as other organic substances (see Table 1) in drinking water, ground water, surface water and treated waste water. The lower application range of this method can vary depending on the sensitivity of the equipment used and the matrix of the sample. For most compounds to which this document applies, the range is e" 0,025 ¼g/l for drinking water, ground water and surface water, and e" 0,050 ¼g/l for treated waste water. The method can be used to determine further organic substances or in other types of water (e.g. process water) provided that accuracy has been tested and verified for each case, and that storage conditions of both samples and reference solutions have been validated. Table 1 shows the substances for which a determination was tested in accordance with the method. Table E.1 provides examples of the determination of other organic substances.