Metal nitrates solvated by octyl(phenyl)–N,N–diisobutylcar-bamoylphosphine oxides exhibit a limited solubility in many alkane diluents even in the presence of a modifier such as tributyl phosphate. The solubility is a nonmonotonous function of the aqueous nitric acid concentration. The solubility of Nd(III) nitrate strongly increases with temperature, branching of the diluent molecule, and the concentration and the molecular weight of a trialkyl phosphate modifier. The solubility also increases when tributyl phosphate is replaced by dibutyl butylphosphonate. Increasing molecular size of n-alkane diluents and branching of the octyl group in the phosphine oxide molecule strongly suppress the solubility. With the n-octyl(phenyl) –N,N–diisobutylcarbamoyl-methylphosphine oxide extractant, the solubility of solvated Nd(III) nitrate is unlimited In the mixed isoparaffinic diluent Isopar–L, and the solvent can be loaded up to a metal to extractant ratio of 1/2. Empirical equations were derived which describe the solubility of Nd(III) nitrate, the distribution ratio of Nd(III), and the distribution ratio of nitric acid as functions of the nitric acid and Nd(III) concentrations in the equilibrium aqueous phase. Empirical equations were also derived which describe the density of the loaded organic phase as a function of the Nd(III) or U(VI) and aqueous nitric acid concentrations.