This International Standard specifies a method for the determination of dalapon, trichloroacetic acid (TCA) and selected haloacetic acids (see Table 1) in ground water and drinking water by gas chromatography (GC-ECD and/or GC-MS detection) after liquid-liquid-extraction and derivatization using diazomethane. Depending on the matrix, the method is applicable to a concentration range from 0,5 μg/l to 10 μg/l. The validated reporting limit of TCA and dalapon is about 0,05 μg/l (see Table C.1). Detection by electron-capture detector (ECD) in general leads to lower detection limits. Detection by mass spectrometry (MS) allows analyte identification. This method may be applicable as well to compounds not mentioned in Table 1 or to other types of water. However, it is necessary to verify the applicability of this method for these special cases.