We show that ellipsometric ic porosimetry can be used for the measurement of the pore size distribution in thin porous films deposited on top of any smooth solid substrate. In this method, in situ ellipsometry is used to determine the amount of adsorptive, which is adsorbed/condensed in the film. Changes in refractive index and film thickness are used to calculate the quantity of adsorptive present in the film. Room temperature porosimetry based on adsorption of vapor of organic solvents has been developed. In this article, a method of calculation of pore size distribution and results of measurements on mesoporous and microporous xerogel films is discussed. Examination of the validity of the Gurvitsch rule fur various organic adsorptives (toluene, heptane, and carbon tetrachloride) is carried out to assess the reliability of measurements of pore size distributions by ellipsometric porosimetry.