Hydrogen diffusion through the palladium-gold (Pd-Au) alloy coatings electrodeposited on Pd (Pd-Au/Pd bilayer symmetric electrode) has been investigated using ac-impedance spectroscopy combined with electrochemical hydrogen permeation method. From fitting the impedance spectra experimentally measured to those theoretically calculated on the basis of derived Faradaic admittance equation for the hydrogen absorption into and diffusion through the bilayer electrode under permeable boundary condition, it is concluded that Au atoms act as trap sites for the hydrogen diffusion through the Pd-Au layer on the one hand and increases the hydrogen overpotential on the Pd-Au layer surface on the other hand. From the comparison between the measured impedance spectra and those simulated as functions of the hydrogen solubility ratio at the Pd-Au/Pd interface and the hydrogen diffusivity in the Pd-Au layer it is deduced that Au atoms impede the hydrogen diffusion through the Pd-Au/Pd bilayer.