An important aspect in understanding these processes is the conformational nature of HSs [6,7]. Depending on circumstances, HSs can form refractory colloids [8], precipitate via aggregation or remain in solution as negatively charged complexes. Therefore, the effect of molecular size and shape of HSs must be taken into account [9]. Much effort has been devoted to the use of size exclusion chromatography (SEC) to answer some fundamental questions about HSs [1,10]. Overcoming artifacts caused by secondary interaction with the column material, high performance size exclusion chromatography (HPSEC) has become a predictive tool for investigation of humic substances [11-14]. Nevertheless, common detection methods (UV-absorption, fluorescence, ICP-MS) have different limitations, such as detection limits, signal quenching and problems caused by online coupling. This prompted us to investigate the possibility of using radioactive tracers in HPSEC. We adapted radiolabeling techniques to label HSs, leading to very good agreement of the radiochromatograms obtained with the results of classical detection methods. In addition to labeling of the carbon backbone of HSs (with 131I), our interest was focused on the interaction of humic substances with aluminum. In the absence of a suitable aluminum isotope, we used 111In as a surrogate for aluminum in our investigations.