The endocrine-disrupting effect of 4-nonylphenols (NP) formed from industrial detergents such as nonylphenol polyethoxylates is well known today. The technical mixture contains a great variety of 4-iso-nonylphenol isomers having different endocrine-disrupting activities. Currently used gas chromatography-mass spectrometry (GC-MS) protocols allow the detection of about 20 peaks, mostly co-eluting isomers. In the present study, Product Ion mass spectrometry obtained by ion trap technology enhanced the selectivity in NP detection resulting in improved gas chromatographic resolution as well as structure assignment of the isomers. The structure proposals of 4-nonylphenol isomers given were derived from GC-MS-MS data processed by multivariate statistics. The cluster analysis allowed the classification of NP due to common structural features that were reflected in the mass spectra. The fragmentation pathways of three reference NP isomers, 4-(1-ethyl-1,4-dimethylpentyl)phenol (NP1), 4-(1,1,5-trimethylhexyl)phenol (NP2) and 4-(1-ethyl-1,3-dimethylpentyl)phenol (NP3), were investigated in more detail. They served as model compounds to aid the interpretation of spectra from unknown NP isomers. Structures of two groups of isomers, characterized by alpha-ethyl as well as alpha-propyl substituents, could be proposed.