The water transport and SO2 crossover in the hybrid sulfur cycle electrolyzer were quantified for a poly(phenylene)-based proton exchange membrane and compared to the performance of industry-standard Nafion membranes. While Nafion exhibits good performance, there exists the possibility of a significant SO2 crossover, which can modify the electrode composition, consume current that should be used for hydrogen production, introduce SO2 to the hydrogen stream, and result in a loss of sulfur from the system. Recent research has focused on poly(phenylene)-based membranes that have exhibited high current density with good stability (both chemical and temperature) while limiting SO2 crossover. In this paper, we extend our previous water-transport-modeling work on Nafion membranes to this polymer electrolyte and directly compare the two in terms of electrolyzer performance and SO2 crossover. We show the ability of poly(phenylene) membranes to operate at elevated temperatures with improved performance over lower temperatures; the high temperature performance exceeds that of Nafion membranes. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3129676] All rights reserved.