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Imaging of the platinum replica of the porous structure and low-pressure argon adsorption allowed us to elucidate the complicated porous structure of SBA-15. These techniques enabled us to draw a coherent picture of the evolution of the SBA-15 precursor mesophase as a function of the synthesis temperature. TEM of the platinum replicas has been unable to show bridges between the structural mesopores of SBA-15 synthesized at low temperature, whereas mesoporous bridges are clearly observed for samples formed at higher temperature. Argon adsorption has evidenced the ultramicroporosity of the materials formed at low temperature, as well as its evolution to secondary porosity with diameters greater than 1.5 nm under more severe hydrothermal treatment.