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Natural organic matter (NOM) is known to play an important role in the transport and binding of trace metal elements in aquatic and soil systems.Thallium is a pollutant for which the extent of the role played by NOM is poorly known.Consequently,this study investigates thallium(I) and its complexation to a purified humic substance as proxy for NOM.Experiments were performed with the Donnan Membrane Technique to separate,for the first time,the free Tl+ ion from its complexed form in the bulk solution.Various pH and concentrations were investigated at constant ionic strength and constant NOM proxy concentrations in solution.Experimental results were described with NICA-Donnan model.Thallium complexation was compared to silver complexation using literature data and using the same NICA-Donnan formalism.Parameters for these two cations (Tl+ and Ag+) are reported in this article,for the first time.Results display low thallium complexation to the NOM proxy while silver competes with divalent cations for the NOM binding sites.Calculated speciation for dissolved thallium highlights the dominance of free thallium (Tl+) in solution whereas Tl-NOM complexes contribute roughly 15％ to total Tl(I) species in river and lake type waters.Similar results are obtained for soil solutions,Tl-bound to NOM ＜ 30％ oftotal,from UK soils with different land use and geochemistry.