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A thermodynamic model that allows one to determine the number and the stoichiometry of the complexes formed between trivalent lanthanide(Ln) nitrates—Pr(NO3)3, Nd(NO3)3, Sm(NO3)3, Eu(NO3)3, Gd(NO3)3, and Yb(NO3)3—and octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide(CMPO) diluted in nitrophenylhexyl ether(NPHE) is presented in this work. The Mikulin–Sergievskii–Dannus'model was used to model the extraction at 25°C of the Ln(NO3)3–H2O–CMPO 0.2 mol kg−1–NPHE system with or without a salting-out agent. The stoichiometric mean activity coefficients of components in the aqueous phase were calculated from experimental data using the Mikulin's equation, whereas the activity coefficients of the species in the organic phase were calculated from the Sergievskii–Dannus'equation. A satisfactory description of the distribution of lanthanide nitrate and water was then obtained over a wide range of lanthanide concentration in the aqueous phase by taking into account the formation of the following complexes:[Ln(NO3)3](CMPO)3(1:3),[Ln(NO3)3](CMPO)2(1:2), and[Ln(NO3)3](CMPO)(1:1). [ABSTRACT FROM AUTHOR]