An attractive method for the characterization of the porosity of thin films is proposed. The adsorption and desorption of vapour are coupled to the measurement of the thickness and electron density of the films by X-ray reflectometry. The variation of the density by vapour adsorption and capillary condensation enables us to determine the amount of adsorbed vapour as a function of the solvent relative pressure and therefore to draw adsorption-desorption isotherms. From these isotherms, the specific surface area, the pore size distribution, the total pore volume as well as the volume fractions of open and closed porosity can be determined. This method allows to overcome the limitation of the classical volumetric adsorption techniques successfully applied for powder or porous but requiring a large amount of sample. Moreover X-ray reflectometry is applicable with usual substrates, as long as the substrate is large enough and its density is different from that of the supported film. The isotherms of sol-gel derived and plasma-enhanced chemical vapour deposited silica coatings are given as examples. (c) 2005 Elsevier B.V. All rights reserved.