The mechanical properties of mesoporous silica films were characterized by x-ray reflectivity measurements. The measurements provide information on the deformation of the pores and the walls induced by the adsorption of water in the pores. The analysis of the nanoscaled deformations supplies a method to determine the elastic modulus E of thin porous films. The nanodeformation of the porous network during its filling with water is interpreted in three regimes of isotherm sorptions.