Members of the new solid solution aluminum phosphite/ethylenediphosphonate series, Al2[(O3PC2H4 PO3)1-x(HPO3)2x] (H2O)2F2·H2O (0 ≤ x ≤ 0.32), have been prepared and fully characterized. The full dehydration behavior of the parent material of this series, Al2[O3-PC2H4 PO3](H2O)2F2 ·H2O (x = 0) has been resolved. On heating the material to 230 °C the extraframework water is the primary species desorbed and the framework structure remains intact as determined from the crystal structure of the partially dehydrated material, Al2[O3-PC2H4 PO3](H2O)2F2 ·0.51(3)H2O. Above 150 °C, but more noticeably above 200 °C, framework water is lost, resulting in the formation of 4- and 5-coordinated Al centers. By 340 °C all the framework water and some fluorine is lost resulting in the collapse of the crystalline material. The phosphite substituted materials (0 <x ≤ 0.32) are shown, by diffraction and spectroscopic techniques, to be single-phase solid solutions. Rietveld refinement of the structure of the x = 0.19 member reveals that a random substitution of phosphite groups for diphosphonate species exists throughout the bulk of the material and that the remainder of the framework remains unaltered by incorporation of this moiety. The readsorption behavior of the materials (0 ≤ x ≤ 0.32) indicates that the temperature at which the extraframework water is first removed decreases as x increases and that the amount of water lost and degree of readsorption increase as x increases. These results indicate that the porosity of the materials can be controlled in a manner conducive to their rational design.