The flexible, anionic, open-framework material NaLa[(PO3H)(2)CH-C6H4-CH(PO3H)(2)]center dot 4H(2)O (NaLa(H4L)) exhibits an exceptional selectivity for monovalent metal cations. This study elucidates the relationship between the ion-exchange behavior and the framework flexibility of this recently discovered material. The exchange of the Na+ ions in NaLa(H4L) with alkaline-earth, alkaline, and selected transition metal ions was studied. EDX and ICP-OES elemental analysis revealed that ion exchange was successful with monovalent ions, while higher-valent ions were rejected. An explanation for this charge selectivity could be found in the site-specific role of the guest cation. X-ray diffraction and thermogravimetric studies on the reversible hydration and dehydration behavior demonstrate that NaLa(H4L) has a flexible framework. Contraction of the channels upon dehydration leads to a decrease in the cell volume of 15%. Rietveld refinement of the structure of the dehydrated form NaLa(H4L)(dehyd) revealed the key role played by the guest cation in the channel-shrinking mechanism. In the hydrated, expanded form, each Na+ ion guest shares three phosphonate oxygens with a La3+ ion in a lanthanum phosphonate chain that defines part of the wall of a rhombic channel. The Na+ ion completes its octahedral coordination sphere with two water molecules and a weaker bond to a fourth phosphonate oxygen. In the dehydrated, contracted form, the Na+ ion loses the two water molecules and moves toward a second La3+ ion, which is located in an adjacent lanthanum phosphonate chain, to share two more phosphonate oxygens, and becomes 5-coordinate. This results in the formation of an -La-O-Na-O-La- chain and a concomitant shrinking of the channels. A comparison of the monovalent metal (M(I)) ion-exchanged compounds, M(I)La(H4L), reveals that both the ionic radius and the enthalpy of hydration of the guest cation affect the equilibrium between the expanded and the contracted forms, and that the framework adapts specifically to the size of the guest cation.