The determination of the porosity of mesoporous silica thin films by X-ray Reflectivity (XR) is reported. The total porosity is deduced from the total reflection plateau and a quantitative analysis involving the calculation of the XR curves by the matrix technique allows to distinguish between micro and meso porosities of the films. Two mesostructured films, in which the surfactant was removed by washing, were investigated. In combination with Grazing Incident Small Angle X-ray Scattering (GISAXS), the surface area of the mesopores is ascertained, thereby providing a complete analysis of the porosity in thin films by X-ray scattering methods.