Electrical and electrochemical properties of different samples of sulfonated poly(ether ether sulfone) (SPEES) membranes with different sulfonation degrees (0%, 5% and 10%) were determined measuring the electrical resistance and concentration potential with the membranes in contact with HCl solutions at different concentrations. Impedance spectroscopy (IS) was used to determine the membrane resistance (R-m) using equivalent circuits as models, and the results show how the sulfonation clearly affects the membrane electrical characteristics, strongly reducing the R-m and also changing the type of circuit associated with the different membranes. The determination of both the proton transport number and the membrane permselectivity to H+ ions in sulfonated samples was performed by measuring concentration potentials (Delta phi). Proton diffusion in the sulfonated membranes were determined from R-m and Delta phi values and a strong reduction in the membrane electrical resistance and an increase in the H+ diffusion coefficient was observed with the increase of the sulfonation degree. A chemical characterization of the surface of nonsulfonated and sulfonated samples was also carried out by X-ray photoelectron spectroscopy (XPS).