The Brunauer-Emmett-Teller (BET) Surface area of 15 nm-thick films made of TiO2,/polyelectrolyte bilayer was determined by quartz crystal microbalance (QCM) measurement of N-2 and Ar adsorption isotherms at 77 K. The measurements were carried out using a home-built vacuum chamber that includes built-in 9 MHz QCM and cryostat units. As little as 1 ng of the adsorbed gas could be detected, and the BET surface area of a flat Au film (ca. 0.5 cm(2)) on an oscillator was determined within an experimental error of +/- 5%. The titania/polymer composite film gives N-2 and Ar adsorption isotherms consisting of a less-pronounced type-I curve and a break at around p/p(0) = 0.7. This behavior is ascribed to the presence of irregular micropores and 6 nm (phi-mesopores in the composite film. An analysis of the isotherms shows that the porosity of the composite film is about 12%. which is much smaller than that of bulk titania (gel powder. The greater density appears to be related to the reported superior properties (robustness and resistance to electrical breakdown) of the organic/inorganic multilayer film. We conclude that the QCM-based, high-precision measurement of Gas adsorption is a powerful tool for investigation of the detailed morphology of nanometer-thick films.