Water diffusion in a sandwich-structured composite film on a Mg-Gd-Y alloy was experimentally investigated using electrochemical impedance spectroscopy (EIS). The composite film contained an inner integration layer and an outer fluorocarbon (FC) layer. The integration layer was prepared by depositing an environment-friendly self-assembled nanoparticle film on the surface of a plasma electrolytic oxidation film. The resulting structure is therefore referred to as PSF. The measured value of the diffusion coefficient of the PSF film by the EIS method was compared to the value predicted using a mathematical model. The noncontinuity of the water concentration at the interface of the integration and FC layers resulted in a significant difference between the diffusion coefficients obtained by the experimental EIS method and mathematical model. A coefficient "k(e)" was introduced to the Fick's first law of diffusion, and a PSF model validation was used to confirm the concentration jump on the FC-integration boundary of the PSF film. When k(e) << 1, the calculated diffusion coefficient (D-eff(M)) of the PSF film by the mathematical model was found similar to that obtained by the EIS experimental method. (C) 2014 The Electrochemical Society. All rights reserved.