Sr2+-UPRM-5 is a titanosilicate containing adjustable structural faulting that prescribes changes in textural properties with temperature. In this work, we studied thermally induced structural changes in Sr2+-UPRM-5 variants prepared using tetrapropylammonium (TPA(+)) and tetrabutylammonium (TBA(+)) and their correlation to the diffusion of CO2 and CH4 at 25 degrees C. Both Sr2+-UPRM-5 materials contained different amounts of structural faulting that are correlated to the formation of 12-MR pores. In situ high-temperature X-ray diffraction revealed structural changes corresponding to orthorhombic phases up to 300 degrees C. Analysis of in situ high-temperature 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy revealed new silicon environments surrounding the archetypical Si(2Si, 2Ti(oct)) and Si(3Si, 1Ti(semioct)) coordination centers. MAS NMR data analysis indicated that the Si environment in Sr2+-UPRM-5 (TPA) appears to be more susceptible to changes upon thermal treatment. A phenomenological volumetric transport model corrected for particle size polydispersity was used to estimate diffusion constants at 25 degrees C in adsorbents preactivated at different temperatures. At the optimal conditions, the CO2/CH4 kinetic selectivities were 41 and 30 for Sr2+-UPRM-5 (TBA) and (TPA), respectively.