An oxidative degradation method was used to study the occurrence of aromatically bound halogens in high molecular weight organic matter isolated from natural waters and bleached-kraft mill effluents. Gas chromatographic analysis of the degradation products revealed that mono- and dichlorinated compounds were present in all analyzed samples. Furthermore, degradation of organic matter from natural and industrial waters resulted in the same chlorinated degradation products, although the relative distribution of these compounds varied strongly with the type of sample. The identification of aromatically bound chlorine in fulvic acids from natural waters represents the first successful determination of structural elements that can explain the widespread occurrence of adsorbable organic halogens (AOX) in unpolluted environments.