The fate of lanthanides (Ln) and actinides (An) +III in the environment is strongly influenced by inorganic and organic colloids including natural organic matter (NOM) [1]. The repartition of the metal between immobile and potentially mobile fractions is mainly driven by pH and NOM to surface concentration ratio. In a system consisting of surface/NOM/metal, it is up to now not clear which compartment influences most An/Ln(III) chemistry. The repartition and chemical environment of europium(III) in the presence of α‐Al 2O3 and humic acid vs. pH and NOM concentration is studied combining batch experiments and spectroscopic characterisation by time-resolved luminescence spectroscopy (TRLS). The fate of Eu(III) seem to be mostly linked to NOM from its chemical environment probed by TRLS. Nevertheless, some subtle differences are evidenced pointing to a modification in the structure of NOM sorbed on oxide surface. 1. McCarthy, J.F., et al.: Environ. Sci. Technol. 32, 3901 (1998).