Thermodynamics of the formation of coordination polymers (CPs) or metal-organic frameworks (MOFs) has not been focused on, whereas many CPs or MOFs have been synthesized in a solution. With a view of separating Nd3+ and Dy3+ in an aqueous solution, we demonstrate that crystallization of the CPs of Nd3+ and Dy3+ based on dibutyl phosphoric acid (Hdbp) can be thermodynamically described; crystallization yields of [Ln(dbp)(3)] (Ln = Nd or Dy) complex are predicted well using a simple calculation, which takes the apparent solubility products (K-sp(a)) for [Ln(dbp)(3)] and the acid dissociation constant of Hdbp into account. The K-sp(a) values of [Nd(dbp)(3)] and [Dy(dbp)(3)] are experimentally determined to be (1.3 +/- 0.1) X 10(-14) and (2.9 +/- 0.4) X 10(-18) M-4, respectively, at 20 degrees C. The ratio of these K-sp(a) values, that is, ca. 4500, is significantly larger than the ratio of the solubility products for inorganic salts of Nd3+ and Dy3+. Therefore, Nd3+ and Dy3+ are selectively crystallized in an aqueous solution via the formation of CPs. Under optimized conditions, Dy3+ crystallization is preferable, whereas Nd3+ remains in the solution phase, where the ratio of the Dy molar content to the total metal content (i.e., Nd + Dy) in the crystal is higher than 0.9. The use of acids, such as HCl or HNO3, has no practical impact on the separation in an aqueous solution.