The mass‐transfer response of a rotating disk electrode to a sine wave modulation of the angular velocity has been calculated for an interface covered by a porous layer of thickness , through which an ionic species is transported by molecular diffusion with a diffusivity . Electrohydrodynamical (EHD) impedances, defined as the transfer functions between the response of the concentration or concentration gradient at the interface and the modulated velocity, have been expressed as functions of a dimensionless frequency p, representing the ratio of the modulation frequency to the average angular velocity, of the permeability and of the diffusion time constant through the film . The frequency analysis of the impedance data for a coated electrode, therefore, allows one to determine and separately.