ISO 17858:2007 specifies a method for the determination of dioxin-like tetra- to hepta-chlorinated biphenyls (PCBs) in waters and wastewaters (containing less than 1 % suspended solids) using high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS). The minimum levels at which the dioxin-like PCBs can typically be determined with no interferences present are specified. 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 method are met. The requirements for establishing method equivalency are given.