The electropolishing of rotating copper-disk electrodes in concentrated phosphoric acid is studied by steady-state measurements, ac-impedance spectroscopy, and now-modulation spectroscopy The effects of applied potential, rotation speed, and temperature on the response of the system are investigated at the mass-transfer-limited plateau. Both the Arrhenius behavior of the limiting current and the ac-impedance data suggest that the rate of reaction is controlled by mass transfer of an acceptor to the copper surface and not by precipitation of a salt film. It is not possible to determine from steady-state measurements the diffusion coefficient of the acceptor because its identity and bulk concentration are not known with certainty. Therefore, the steady-state measurements are complemented with flow-modulation spectra which allow an estimation of the diffusion coefficient of the acceptor without knowledge of its bulk concentration.