Humic substances are known to be a potential vector for the transport of trivalent metal ions in the environment due to non-reversible interactions. The goal of the present paper is to understand the origin of this non-reversibility using polyacrylic acid, PAA, as model substance and Eu(III). The reversibilty of the interaction between Eu(III) and two polyacrylic acids (5000 and 50000 Da) is investigated by comparing the interaction constants deduced from complexation and dissociation experiments in the presence of the chelex resin. The work is done at pH=5 in 0.1 mol L–1 NaClO4. Fluorescence spectroscopy is used as a speciation method to characterize the mode of interaction between Eu(III) and PAA functional groups. The results show that Eu(III), in a first step, interacts rapidly with polyacrylic acid functional groups, probably by electrostatic forces. This process is followed by a rearrangement of the polymer chain to form an inner sphere site where Eu(III) is probably coordinated with three carboxylate groups. The interaction between Eu(III) and polyacrylic acid is shown to be reversible: the parameters deduced from complexation and dissociation experiments are identical. This result differs from that previously reported for the model system composed of synthetic polycarboxylic acids and Th(IV). This discrepancy is explained by the difference in the nature of the metal ion studied and/or by the difference in the range of ligand-to-metal ratios investigated. The difference in complexation behavior of Eu(III) with natural and synthetic polyelectrolytes is ascribed to their different structures.