Mesoporous silica films of thickness 0.1-0.2 mu m supported on glass and silica wafer substrates were fabricated by sol-gel coating method through P123 and Brij-56 surfactant templating synthesis. Their calcined forms were found to exhibit high porosity (39-50%) and a channel structure with uniform pore size in the range of 2.6-5.0 nm. The hydrophilic mesoporous silica films after calcination were further modified to achieve an acceptable hydrophobicity through in-situ TMCS silylation during the preparation of precursor sols and post-treatment with HMDS vapor. The density of calcined mesostructured films with constant thickness was found to increase with the environmental humidity due to water uptake in the intrachannel void space. Silylation treatment could reduce the density increase as detected by specular X-ray reflectivity. The thermal stability of such enhanced hydrophobicity up to 400 degrees C and the reaction of TMCS and HMDS with the surface silanol group to form hydrophobic -Si(CH3)(3) were investigated by infrared and temperature-programmed desorption with mass spectroscopy. Reaction details of the hydrophobic modification of mesoporous silica by TMCS were also studied by Si-29 NMR. (c) 2005 Published by Elsevier B.V.