The permeation of water and corrosive species in coating films is very important for the corrosion protection by organic coatings. As a new method for the study of diffusion of media in coating films, bulk. acoustic wave (BAW) admittance analysis has been proposed in this article. Real-time information on the change of the him mass, mechanic property of the layer, and the permeation process of the media can be provided simultaneously by the parameters of BAW admittance analysis, f(s) (series resonant frequency), Delta L-1 (inductance variance), R-1 (resistance), Q (quality factor), and C-s (film capacitance). The diffusion behavior of media and the corresponding film stability can be described by the following parameters : D (diffusion coefficient), P (permeation coefficient), S (the solubility of media), and SC (the swelling coefficient of the coating layer).