SiO2 thin films prepared by the sol-gel process from TEOS as alkoxide precursors and Triton X45 as additive have been investigated by vibrational spectroscopy, thermal analysis, and N-2 adsorption/desorption measurements. These studies indicate that microporous materials capable of retaining both high surface area and free silanol concentration at 600 degrees C were obtained. These findings are discussed in relation to the structure of the surface silanols, interaction between silanols, and the additive and the thermal degradation of the latter. On these bases a formation mechanism of the porous structure is proposed.