X‐ray reflectivity porosimetry is a highly sensitive measurement method that utilizes the capillary condensation of a solvent vapor inside porous low‐k dielectric films on a silicon substrate. As the partial pressure of the solvent environment over the film increases, capillary condensation occurs in progressively larger pores. This results in an appreciable increase in the electron density of the film. By monitoring the changes in the critical angle for total X‐ray reflectance, one can directly calculate the average electron density, and therefore the solvent uptake. By invoking traditional porosimetry absorption/desorption analyses, characteristics such as porosity and the distribution of pore sizes can be extracted. © 2003 American Institute of Physics