The oxidative polymerization of 1-naphthol was investigated in the presence of horseradish peroxidase (HRP). Naphthol polymerization products (NPP) were characterized for their relative polarity using octanol--water partitioning experiments and reverse-phase high pressure liquid chromatography, for structure using size exclusion chromatography and liquid chromatography-mass spectrometry (LC/MS), and for ecotoxicity using inhibition of bacterial bioluminescence. Peroxidase addition resulted in the production of soluble and insoluble NPP. Soluble NPP was predominantly more polar than the parent naphthol and comprised of trimers and tetramers. Insoluble NPP oligomers included dimers, trimers and tetramers. The net aqueous-phase toxicity was significantly reduced due to polymer formation and subsequent precipitation. A reaction model deduced from the LC/MS fragmentation patterns of trimeric naphthol was proposed for NPP formation. Results from this study suggest that HRP-mediated treatment of naphthol contaminated soils can achieve risk reduction through (i) the formation of large hydrophobic oligomers that are immobilized on the soil matrix; and (ii) reduction in aqueous-phase toxicity due to polymer precipitation.