Natural organic matter (NOM) affects the fate of radionuclides in the environment, either by supporting their mobility in water, or by limiting their migration in soils and sediments. Eu(III) was studied as a chemical analogue for actinides(III). Batch experiments were done to build complexation isotherms at different Eu(III) concentrations and pH, using Suwannee River fulvic acid (SRFA) concentrations up to 1 g/L as a proxy for NOM reactivity. Eu(III) speciation was investigated by time-resolved luminescence spectroscopy. Two different luminescence behaviours of Eu(III) were observed (Fig. 1): (i) the first part of the isotherms at low C(SRFA) is showing the typical luminescence evolution of Eu(III) complexed by humic substances [1]; and (ii) at higher concentration (C > 100 mgSRFA/L at pH 4, C > 30 mgSRFA/L at pH 6 and 7), a second luminescence mode is detected and could correspond to a different spatial organization of the complexed europium. To better understand this second mode, we performed experiments at various ionic strengths. The complexation is typically decreasing with ionic strength in the first part of the isotherm [2,3], whilst the opposite influence was shown in the second part of the isotherm. Luminescence decay times are also showing distinctive evolutions. The fulvic and ionic strength effects evidenced spectroscopically suggest that in addition to intra-particulate complexation mode (first complexation-edge), there might be inter-particulate repulsion between fulvic acid particles that are complexing Eu(III) in the second part of the isotherm, which is not yet accounted within the different complexation models. The account of an interfacial potential for fulvic acid particles, or a Donnan volume depending on the hydrodynamic radius of SRFA was proposed [4,5], and could be considered at high fulvic acid concentration.