The current frequency response at a rotating disk electrode with a non-uniform reactivity is analysed for two types of sinusoidal perturbation: on the one hand, the angular speed is modulated, the potential being fixed (EHD impedance), and on the other hand, the electrode potential is modulated, the speed being fixed (usual ac impedance). In the low frequency range, the response behaviour corresponds to that of a uniform accessible disk electrode. In the high frequency range, its response is similar to the response of an isolated active site. The frequency lag between the two regimes is related to the dimension of the active site and to its radial coordinate.