The electrochemical impedance spectra of MgMnCO3 LDH films oxidized at different dc potentials were recorded. The results were fitted to a Randles type cell by replacing the Warburg impedance with a mass transfer resistance in parallel with a constant phase element. The films charge transfer resistances decreased dramatically at the onset of manganese oxidation. In thin films, R ct decreased from 104 Ω for an un-oxidized film to a minimum of 40Ω in a film oxidized at 0.32V, before increasing back to 104 Ω in a film oxidized at 0.5V. Iodometry measurements show these changes correspond to increases in the manganese average valence in the films from 3.09+ prior to oxidation, to 3.80+ at 0.32V and 3.95+ at 0.5V. In thicker films, however, a much higher dc potential, 1.0V, was required to return R ct to 104 Ω. There was also less change in the manganese average valence in the thicker films. Oxidation at 1.0V only increased the manganese valence to 3.33+. For the partially oxidized films, the Nyquist plots consisted of depressed semicircles at high frequency, followed by linear regions at lower frequency where the impedance was controlled by mass transport. The effective diffusion coefficient estimated from the low frequency impedance was 1×10−9 cm2 s−1, consistent with proton diffusion in solid electrodes. The impedance spectrum of a partially oxidized film reduced at −0.2V was similar to that of the un-oxidized film.