This International Standard specifies a method for the determination of tetra- to octa-chlorinated dibenzo-pdioxins (PCDDs) and dibenzofurans (PCDFs) in waters and waste waters (containing less than 1 % by mass solids) using high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS). This International Standard is applicable to the seventeen 2,3,7,8-substituted PCDDs/PCDFs specified in Table 1. The detection limits and quantitation levels in this method are usually dependent on the level of interferences rather than instrumental limitations. The minimum levels (MLs) specified in Table 2 are the levels at which the PCDDs/PCDFs can be determined with no interferences present. The method detection limit (MDL) for 2,3,7,8-TCDD has been determined as 4,4 pg/l based on this method using a sample volume of 1 l. Lower detection limits may be achieved by using a larger sample volume. This method is “performance based”. The analyst is permitted to modify the method to overcome interferences or lower the cost of measurements, provided that all performance criteria in this International Standard are met. The requirements for establishing method equivalency are given in 9.1.2.