The hydrolytic stability of lanthanide and actinide selective mono- and polyphosphonate-functionalized mesoporous zirconium titanium oxide adsorbents has been investigated in nitric acid solutions. Hydrolytic degradation of the surfaces, as measured through the fractional loss of phosphorus and elements of the oxide framework, increased by more than an order of magnitude as the nitric acid concentration was increased from 0 to 2 mol/L. The unfunctionalized parent oxide suffered considerable dissolution in 2 mol/L acid over a period of 72 h. Under identical conditions, the fractional Zr and Ti release was reduced to 1 × 10(-2) for monophosphonate functionalized hybrids and reached as low as 1 × 10(-6) for trisphosphonate functionalized variants. The bisphosphonates showed intermediate values. The leaching of P, Zr and Ti was found to be incongruent with the Zr leaching to a lesser extent implying enhanced stability of the Zr-O-P bond. Quantitative analysis of the dissolution kinetics indicated a parabolic dissolution model with a rate constant in the range of 0.5-1.5 mg g(-1) min(-1/2) for the elemental leaching of P, Ti, and Zr. The leaching of Zr from the mesoporous matrix was relatively more complex than for the other elements with evidence of a leaching mechanism involving two processes. ToF-SIMS and DRIFT analysis demonstrated that after leaching in 2 M HNO3 for 24 h, a significant proportion of grafted ligands remained on the surface. The oxide functionalized with amino trismethylenephosphonic acid, which had previously shown excellent (153)Gd(3+) selectivity, was demonstrated to have outstanding stability, with low fractional elemental losses and preservation of mesoporous texture even after leaching for 24 h in 2 M HNO3. This suggests this particular hybrid to be worthy of additional study.