This International Standard specifies a method for the determination of selected individual isomers of nonylphenol in non-filtered samples of drinking water, waste water, ground water and surface water. The method is applicable in concentrations between 0,001 µg/l and 0,1 µg/l for individual isomers and from 0,01 µg/l to 0,2 µg/l for the sum of 4-nonylphenol (mixture of isomers). Depending on the matrix, the method is also applicable to waste water in concentrations between 0,1 µg/l and 50 µg/l.