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ISO/CD 18857-2 (International Organization for Standardization, Geneva) describes a now international standard method for the determination of octylpbenol, nonylphenol, their mono- and diethyoxylates, and bisphenol A in nonfiltered samples of drinking ground, surface, and wastewater. The method is based on the extraction of the analytes from an acidified water sample by solid phase extraction, solvent elution, derivatizafion, and determination by gas chromatography with mass spectrometric detection. For validation of this method, 14 laboratories from 4 different countries in Europe and Canada participated in an interlaboratory trial to determine the performance characteristics of the method, which are intended for publication in the corresponding standard. The interlaboratory trial was evaluated according to ISO 5725-2 and included two duplicate nonfiltered water samples: surface water containing the target compounds in an analyte concentration range from 0.05 to 0.4 [micro]g/L and wastewater containing the target compounds in a concentration ranged from 0.1 to 5 [micro]g/L. The repeatability variation coefficients (within-laboratory precision) varied for all samples and compounds between 1.9 and 7.8%, showing a sufficiently high repeatability of the method. The reproducibility variation coefficients (between-laboratory precision) were found to vary within a satisfactory range of 10.0-29.5% for surface water and 10.8-22.5% for wastewater. The recoveries as a measure of accuracy varied from 98.0 to 144.1% for surface water and from 95.4 to 108.6% for wastewater. The determined concentrations of the samples compared well to the 'true' values, thus showing very satisfactory accuracy of the method. In the chromatogram of the surface water sample, a high unresolved background made up of coextractable matrix compounds was apparent. It is conceivable that compounds from tiffs background may be responsible for enhanced recoveries of 144.1% for 4-nonyiphenol (mixture of isomers) and of 123.4% for 4-nonyiphenol monoethoxylate (mixture of isomers) in the surface water samples. The isotope-marked standard compounds developed in this context proved to be reliable internal standards that allow a precise and accurate quanfitation of all compounds specified in ISO/CD 18857-2. The results of the interlaboratory trial cenfumed that the analytical method is robust and reliable and can be used as a standard method to analyze the target compounds in water samples.