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A model for describing the diffusion behavior of water vapor in barrier films is developed and applied to the cases of 1) pure polyethylene terephthalate (PET); 2) an inorganic thin film (ITF) with defects on a PET substrate (ITF/PET); and 3) PET/ITF/PET. The normalized concentration profile and the relative mass of water vapor in the ITF/PET and PET/ITF/PET barrier films are simulated as a function of diffusion time by considering the sorption and desorption of water vapor. The number of atoms penetrating the barrier films is quantitatively evaluated by the concentration gradient of water vapor, which is related to the water vapor transmission rate (WVTR). The simulated WVTR values in pure PET, PET/SiOx, and PET/SiOx/ORM layered structures are in good agreement with the experimental ones.