Electrochemical impedance spectroscopy (EIS) was used to measure the solid-state diffusion coefficient of protons in nickel hydroxide films at room temperature as a function of state of charge (SOC). A model for the complex faradaic impedance of the nickel hydroxide active material is presented and used to extract the diffusion coefficient of protons from the EIS data. Impedance data over a range of frequencies can be used to extract a constant diffusion coefficient without the knowledge of the initial mobile proton concentration or the form of the charge-transfer kinetic expression. The proton diffusion coefficient is a strong function of SOC and decreases approximately three orders of magnitude from 3.4 X 10(-8) to 6.4 X 10(-11) cm(2) s(-1) as the electrode discharges from the completely charged to the completely discharged state. The measurements were performed on well-conditioned nickel hydroxide films and therefore it is likely that the diffusion coefficients measured correspond to the p-phase of the active material. The diffusion coefficient of protons was measured for three different film thicknesses, 1.5, 1.2, and 1.0 mu m. The diffusion coefficient is independent of the thickness of the film as predicted by theory. The three orders of magnitude decrease in the diffusion coefficient of protons can be explained on the assumption that the protons move predominantly through the oxidized phase [NiOOH] which is interdispersed along with the reduced phase [Ni(OH)(2)] in the active material.