The frequency response analysis for an electrochemical system (current or potential) to a sinusoidal speed modulation at a rotating disk electrode involves, at first, the analysis of the corresponding unsteady laminar flow. This hydrodynamic problem is solved by a numerical integration of the unsteady Navier‐Stokes equations, and the four first terms of the instantaneous velocity expansions are given. The study of the unsteady mass transport is the second problem; its solution is a series expansion of , and the two first terms are given. Finally, the expressions of the electrical response of the electrochemical system involves also the element of the electrochemical impedance (the diffusion or convective Warburg impedance, the charge transfer resistance, the electrolyte resistance, and the double‐layer capacitance). The results are experimentally confirmed, in potentiostatic and galvanostatic regulations, over a wide frequency range for the reduction of to in .