Polarization curves of fuel cells incorporating PFSA short-side chain (SSC) ionomer membranes having ion exchange capacity (IEC) 1.30, 1.37, 1.43 and 1.50 meq g(-1) and NRE-211 are compared. Under low humidity conditions, fuel cells incorporating SSC membranes show higher performance than NRE-211. SSC PFSA membranes possessing an IEC of 1.37 meq g(-1) exhibit the highest performance. Differences in fuel cell polarization curves are due to differences in the high frequency resistance, which in turn is found related to ex-situ measurements of both the effective proton mobility and the rate of water flux through the membrane, which also exhibit a maximum for membranes of IEC 1.37 meq g(-1). Water permeation and proton mobility are shown to be inherently linked, but it is found that simply increasing the membrane's IEC does not necessarily translate to increased water transport and effective proton mobility, despite the increased water content. (C) 2013 Crown Copyright and Elsevier Inc. All rights reserved.