[en] The understanding of radioelement behaviour in natural systems in relation with nuclear waste disposals in geological formations necessitates the knowledge of their speciation in these systems. In particular, this implies to determine the influence of humic substances (humic and fulvic acids, HA/FA) as natural organic substances present at different concentrations in groundwater on the migration of radionuclides, particularly actinide elements. This induces in particular to study the complexation of actinides with humic substances as complexing agents. Hence, our objective is to obtain data (interaction constants, complexing capacities) on the interactions between humic/fulvic acids and actinides under relevant geochemical conditions (pH, ionic strength, presence of competing cations). Moreover, it is also important to focus on the possible existence of mixed complexes (or ternary), namely M-OH/CO₃-HA/FA, which will then completely modify the actinide speciation compared to the absence of such complexes. The technique retained to study such complexes is time-resolved laser-induced fluorescence, which has been used, up to now, by both CEA laboratories for the study of trivalent actinides and lanthanides for pH<7, and for the study of mixed complexes in the case of uranium (Moulin et al., 1999a) The approach developed for U(VI) for the investigation of mixed complexes is applied in the case of europium, which has been chosen as a chemical analogue of trivalent actinides and which can be analysed at low level by TRLIF (Bador et al., 1989, Decambox et al., 1989). Its hypersensitive transition is a relevant indicator of the complexation phenomena (Horrocks et al., 1979, Dobbs et al., 1989, Moulin et al., 1999 b). Firstly, a spectrum data base concerning the different complexes (hydroxide, carbonate and humate) is obtained on model systems with all spectroscopic characteristics (lifetimes, fluorescence wavelengths). And, secondly, from these data, titrations of europium solutions by humic substances are carried out at fixed pH and ionic strength and at atmospheric pressure. (orig.)