The behaviour of a monoalkyl quaternary ammonium antioxidant in seawater (0-34 psu) has been studied. A simulating study of quaternary ammonium salt (QAS) in an environmental setting was carried out by use of two different compound compositions: pure QAS (QAS-A), and a mixture of QAS and fatty acid amine dissolved in propanol. The study shows that the presence of a hydrophobic constituent (fatty acid amine) might have an influence in the physical property of the antioxidant in aquatic medium. The QAS-A shows uniform solubility towards seawater of different salinity. The combined material flocculates at approximately 1% V/V in seawater. When filtering off the flocculated matter, the compound show a solubility similar to the QAS-A. The investigated QAS have high affinity towards natural sediment from Vestfjorden, Oslofjorden, Norway, but different adsorbing mechanisms seem to be found when comparing the pure and the combined material. A further adsorption/desorption study of the combined material show that this have an increased affinity towards sediment with organic content (~2% TOC), compared to organic free sediment. When UV-irradiated the combined material show indication of break down by fragmentation rather then mineralization. It was not possible to identify the products from UV break down by UV-, IR- or fluorescence spectroscopy.