Communication: Very stable surface gratings can be inscribed in azo-hybrid thin films prepared by the sol-gel method, as described here. It is shown that in order to obtain high modulation depths a weakly crosslinked network in the gel with a degree of condensation that can be easily controlled by heat treatment and the functionality of the sol-gel precursor is required. In addition, the simultaneous presence of an intensity gradient and a non-zero component of the resultant optical field along this gradient is required. Explanations for the mass transport mechanism are also proposed.