This study aims to investigate and model the adsorption of Eu(III) on bentonite in the presence of phosphate. Binary (phosphate/bentonite) and ternary (Eu(III)/phosphate/bentonite) systems were studied as a function of contact time, pH, solid-to-liquid ratio and Eu(III)/phosphate concentration by using a batch experimental method. The adsorption of phosphate on bentonite slightly increased in the pH range of 2.5-6.5, and decreased in the pH range of 6.5-9.4. This adsorption can be quantitatively interpreted by a model considering the formation of three monodentate surface complexes. In the ternary system, a synergistic adsorption was observed in the presence of both phosphate and Eu(III). In addition to the two sub-models describing Eu(III) and phosphate adsorption, the formation of ternary surface complexes had to be considered in order to explain the synergistic effect experimentally observed. The experimental data could be quantitatively explained when Eu(III) (6-point triple bond; length half of m-dashSOEuH2PO4+ and 6-point triple bond; length half of m-dashSOEuHPO4) or phosphate (6-point triple bond; length half of m-dashSPO4Eu+) are the bridged atoms. Complementary experiments carried out by X-ray photoelectron spectroscopy suggested that the second case is the most probable. The proposed model can be used in order to predict Eu(III) adsorption on buffer/backfilling material in the presence of phosphate.