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Sr 2+ -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 Sr 2+ -UPRM-5 variants prepared using tetrapropylammonium (TPA + ) and tetrabutylammonium (TBA + ) and their correlation to the diffusion of CO 2 and CH 4 at 25°C. Both Sr 2+ -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°C. Analysis of in situ high-temperature 29 Si 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 Sr 2+ -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°C in adsorbents preactivated at different temperatures. At the optimal conditions, the CO 2 / CH 4 kinetic selectivities were 41 and 30 for Sr 2+ -UPRM-5 (TBA) and (TPA), respectively.