This paper reports the electrochemical characteristics of phosphonic and sulfonic acid membranes based on commercial polyphenyl sulfone Radel. Proton membranes' permselectivity was determined from the electromotive force of concentration cells measured with silver/silver chloride electrodes reversible to the anions of the electrolyte solutions. Apparent proton transport numbers in most membranes are close to the unit, even at moderately high electrolyte concentrations. The water uptake of polyphenyl sulfone phosphonic acid membranes is lower than that of membranes containing sulfonic acid groups anchored to their structure and, as a result, the former membranes have a lower conductivity than the latter ones with a similar cation-exchange capacity. For example, the conductivity of the sulfonic acid membrane with an ion-exchange capacity of 2.06 meq/g dry membrane, equilibrated with water, is 3.56x10(-4) S/cm at 25 degrees C, about one order of magnitude higher than that of the phosphonic acid membrane with an ion-exchange capacity of 1.90 meq/g dry membrane, measured in the same conditions. The diffusion coefficient of protons in the membranes equilibrated with distilled water was calculated and compared with that found for Nafion 117 in the same conditions. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2988043] All rights reserved.