A porosimetry technique that uses infrared-spectroscopic quantification of adsorbed probe molecules has been developed to characterize porous thin films. The amount of adsorbed probe molecules as a function of relative pressure of vapor is quantified via the absorbance of a characteristic signal of the probe molecule, to obtain adsorption isotherms. The pore size distributions of nanoporous thin films of SiO2-ZrO2, TiO2, and mesoporous silica MCM-41 are determined based on the Kelvin equation. In this technique, water and hexane are used as condensable vapors. Pore size distribution curves measured by infrared-spectroscopic porosimetry are in good agreement with those measured by the conventional gas/vapor adsorption technique, although the surface area per sample was less than 1 square meter. The results verify that the infrared-spectroscopic porosimetry technique is a simple methodology for characterizing the porous structures of thin films.