The proton diffusion coefficient in γ-MnO2 at various stages of discharge of an electrolytic manganese dioxide electrode has been determined taking into consideration the true molar volume and the electrochemically accessible surface area of MnO2. The unique electrochemical cell which allows the electrode expansion during the discharge to be monitored in situ was reported. The a.c. impedance technique and the transmission line equivalent circuit were used. Electrode ohmic resistance, Faradaic resistance, and double-layer capacitance were also obtained by means of numerical fitting of the impedance data using the transmission line model.