In this paper the exchange of lanthanide(III) ions (Ln3(+)) between a solution and a coordination polymer (CP) of di(2-ethylhexyl)phosphoric acid (Hdehp), [Ln(dehp)(3)], is studied. Kinetic and selectivity studies suggest that a polymeric network of [Ln(dehp)(3)] has different characteristics than the corresponding monomeric complex. The reaction rate is remarkably slow and requires over 600 h to reach in nearly equilibrium, and this can be explained by the polymeric crystalline structure and high valency of Ln3(+). The affinity of the exchange reaction reaches a maximum with the Ln3(+) possessing an ionic radius 7% smaller than that of the central Ln3(+), therefore, the affinity of the [Ln(dehp)(3)] is tunable based on the choice of the central metal ion. Such unique affinity, which differs from the monomeric complex, can be explained by two factors: the coordination preference and steric strain caused by the polymeric structure. The latter likely becomes predominant for Ln(3+) exchange when the ionic radius of the ion in solution is smaller than the original Ln(3+) by more than 7%. Structural studies suggest that the incoming Ln(3+) forms a new phase though an exchange reaction, and this could plausibly cause the structural strain. (C) 2013 Elsevier Inc. All rights reserved.