Nonylphenols (NP) are a mixture of numerous structural and stereoisomeric chemical compounds. The molecular formula (C15H24O) and molar mass (220.35 Da) of the individual compounds are the same. Each individual compound consists of an aromatic ring with hydroxy group and an alkyl radical of nine carbon atoms, usually in para position to the phenolic hydroxy group. The isomer with a linear side chain is called n-nonylphenol and is only of minor industrial importance. Nonylphenols belong to the group of long chain alkylphenols - LCAPs (engl. long chain alkyl penols) - and do not occur naturally in the environment. Because of their estrogenic, persistent and toxic properties, nonylphenols are classified as priority hazardous substances in the EU Water Framework Directive.