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Molecular dynamics simulations have been performed in water on europium nitrate complexes Eu(NO 3 ) 3 L m with three neutral organophosphorus extractants (L = tri-n-butylphosphate (TBP), triphenylphosphine oxide (TPPO) and carbamoylphosphine oxide (CMPO)) in order to determine which factors govern the experimental 1 : 3 stoichiometry in organic solution saturated with water. This stoichiometry was investigated by progressive saturation of the cation coordination sphere by L. Simulations without constraints lead to monodentate coordination of the NO 3 Ϫ anions to Eu 3ϩ , and to stoichiometries higher than 1 : 3. When the NO 3 Ϫ ions are constrained to bind Eu 3ϩ in a bidentate mode, complexes are stable up to 1 : 3 stoichiometry with L = TPPO and CMPO. From the 1 : 3 stoichiometry with bidentate nitrates, the interactions of the cation with water become repulsive, which facilitates the extraction from an aqueous to an organic phase. With TBP as ligand, 1 : 4 stoichiometry cannot be precluded, but formation of this complex appears to be kinetically more difficult than for the 1 : 3 complex.