Periodic mesoporous materials possess high surface to volume ratio and nano-scale sized pores, making them potential candidates for heterogeneous catalysis, ion exchange, gas sensing and other applications. In this study, we use in situ small angle x-ray scattering (SAXS) and molecular dynamics (MD) simulations to investigate the mechanical and hydrothermal stability properties of periodic mesoporous SBA-15 silica and SBA-15 type aluminosilica (Al-SBA-15) to extreme conditions. The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica possess amorphous frameworks and have similar pore size distribution (pore size ∼9-10 nm). The in situ SAXS measurements were made at the B1 beamline, at the Cornell High Energy Synchrotron Source (CHESS). The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica specimens were loaded in a diamond anvil cell (DAC) for pressure measurements, and, separately, with water in the DAC for hydrothermal measurements to high P-T conditions (to 255 °C and ∼ 114 MPa). Analyses of the pressure-dependent SAXS data show that the mesoporous Al-SBA-15 aluminosilica is substantially more mechanically stable than the SBA-15 silica. Hydrothermal measurements show a small net swelling of the framework at elevated P-T conditions, due to dissolution of water into the pore walls. Under elevated P-T conditions, the Al-SBA-15 aluminosilica shows significantly greater hydrothermal stability than the SBA-15 silica. Our MD simulations show that the bulk modulus value of periodic mesoporous SBA-15 silica varies exponentially with percentage porosity. Molecular dynamics simulations are being made in order to better understand how the pore architecture and the chemical composition of the host structure govern the stability properties of the mesoporous materials.