A method for the determination of 18 isomeric 4-nonylphenols and 4-tert.-octylphenol in wastewater using GC-MS and LC-MS has been developed. This procedure has been applied to the determination of the free allylphenols and the analysis of these substances in the form of 4-alkylphenol polyethoxylates, their various hydrosoluble metabolites and other hydrosoluble 4-alkylphenol containing degradation products ("bonded" alkylphenols) after their cleavage with hydroiodic acid. In the environment, the final degradation products of 4-alkylphenol polyethoxylates and their metabolites are the long-chain free 4-alkylphenols, which are responsible of endocrine disruption in various animal species. The average concentration of free alkylphenols in the wastewater of the sewage plant in Aïre, Geneva (Switzerland) ranges from 1.0 to 6.8 microg/l (average 2.5 microg/l). The concentration of "bonded" 4-alkylphenols can reach about 0.66 mg/l. The precision of the method and its accuracy are satisfactory with recovery rates for the free 4-alkylphenols and "bonded" 4-nonylphenols ranging from 74 to 79% and 80 to 87%, respectively. The relative standard deviation is lower than 6% for all analyzed compounds. The detection limits are in the range of 0.4 to 6 ng/l (typically 2 ng/l) and quantification limits are between 2 to 22 ng/l (typically 10 ng/l) for all individual isomeric alkylphenols.