Organically functionalized mesoporous silica films were prepared by a novel synthesis procedure. The synthesis procedure involves a vapor infiltration technique for mesostructured silica films prepared through a spin-coating process. Organic-functional groups were incorporated into the mesochannel wall by the vapor infiltration treatment using organosiloxanes. The structural stability of spin-on mesostructured silica films is generally low, resulting in a structural shrinkage or a collapse of the mesostructure under calcination process. On the other hand, the treated films did not contract during calcination, indicating high structural stability. A contact angle of water on the treated film was large, showing high hydrophobicity. FTIR analysis suggested that the residual silanol groups were replaced with the organosilyl groups after vapor infiltration treatment. The increase in the film thickness was hardly observed under vapor infiltration, suggesting no deposition of organosiloxanes vapor on the surface of the film.