Two structurally related perfluorinated ionomer materials, one a conventional sulfonic-acid-based ionomer (Nafion) and the other an experimental bis[(perfluoroalkyl)sulfonyl]imide-based ionomer in which the sulfonic acid group has been replaced by a sulfonyl imide acid group, were studied in parallel to evaluate their relative utility as membrane materials for use in polymer electrolyte membrane (PEM) fuel cells. Studies focused on membrane ionic conductivity and water content under varying conditions of relative humidity, and on device-level fuel-cell tests using membrane-electrode assemblies (MEAs) fabricated from membranes of the two ionomers. The overall finding is that the two ionomer materials behave similarly with respect to their electrochemical properties and performance in PEM fuel-cell devices. In one instance, a sulfonyl-imide-based MEA exhibited substantially improved performance relative to a comparable Nafion-based MEA in fuel-cell tests. The improvement is probably attributable to a combination of favorable materials properties and membrane thickness effects.