Two mesostructured MCM-41 silicas that differ dramatically in hydrothermal stability have been examined by Si-29 MAS NMR spectroscopy and pair distribution function (PDF) analysis of synchrotron X-ray scattering data. The less stable mesostructure assembled from sodium silicate and the substantially more stable derivative made from fumed silica possess equivalent local framework wall structures, as judged by NMR and PDF methods. Approximately 80% of the SiO4 tetrahedra are fully cross-linked as a 4 (Si(OSi)(4)) units in both calcined samples. Additionally, the structural correlation distances for the two materials are nearly identical, having values of 1.62(1), similar to2.60, and 3.09(1) Angstrom for the Si-O, O-O, and nearest neighbor Si-Si distances in the framework. Sodium ions in the framework play a crucial role in limiting the hydrothermal stability of the mesostructure. Residual sodium (similar to0.05-0.10% Na2O) is retained in the MCM-41 made from sodium silicate, even after two ion exchange reactions with ammonium ions in more than 300 - fold excess. The entrapped framework sodium ions catalyze the collapse of the mesopores upon exposure to 20% steam at 800 C for 5 h. The sodium - free mesostructure assembled from fumed silica retains an open framework under the same hydrothermal conditions. The stability of the fumed silica derivative, however, is greatly compromised when doped with as little as 0.10% Na2O, thus confirming the deleterious effect of sodium on hydrothermal stability.