The type of association between pollutants and humic substances of soils, sediments and river waters has been investigated. Metabolites, which can arise from the microbiological degradation of polyaromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB), were cleaved from the macromolecular matrix by selective chemical degradation techniques (OH^−, BCl₃, Rh/H₂). Hydrolysis reactions performed with Na¹⁸OH proved that some metabolites of pollutants form stable ester bonds by condensation processes with functional groups of humic substances, a phenomenon which has major implications for transport, toxicity and bioavailability of xenobiotica.