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International audience; Interactions between Natural Organic Matter (NOM) and mineral surfaces modify the properties of both reacting phases. It also alters the behaviour of metal ions such as radionuclides in the environment. Here, we focus on the influence of the interactions between humic substances, major NOM components, and some minerals of soils (metal oxides and clays) on the speciation of europium (III), chosen for its analogy with trivalent actinides. To understand the binding of metal ions, proton binding must first be considered. Potentiometric titrations of both organic matter and mineral suspensions are realized before and after their interactions, to study variations of their reactivity. To be closer to environmental conditions (i.e. low DOC concentrations), a new approach must be considered to characterize organic matter reactivity. Spectrophotometric titrations and UV-vis spectra of NOM [1] are used to estimate changes in reactivity with the help of differential absorbance spectra variations of the solution as function of pH. The correlation between the two methods of characterization of NOM is quite good, and it is used to determine the changes of the organic matter properties after adsorption onto the mineral surfaces, in order to better describe the binding of radionuclides in a ternary system. The speciation of Eu(III) in presence of NOM or oxide surface is also studied by time resolved laser fluorescence spectroscopy (TRLFS). [1] Dryer, Korshin & Fabbricino (2008) Environ. Sci. Technol. 42, 6644-6649.