In this work, chemical degradation is studied using highly controlled measurements of the fluoride ion release from subscale cells in degrading environments using perfluorosulfonic-acid-based membrane electrode assemblies, primarily with cast, 25 mu m (1 mil) thick membranes. Effects of key variables, such as oxygen concentration, relative humidity (RH), temperature, and membrane thickness on the fluoride ion emission rate (FER) are described under open-circuit decay conditions. Some of the observed trends are expected or consistent with previous observations, such as decreasing FER with decreasing temperature and increasing RH. Other trends observed are not expected, such as a logarithmic decrease of FER with oxygen concentration and increasing FER with increasing membrane thickness. Cross-sectional transmission electron microscopy analysis of decayed membranes indicates a surprisingly homogeneous distribution of small Pt particles (similar to 3 to 20 nm in diameter), presumably from dissolution and migration from the cathode. The experimental results are consistent with radical generation at these Pt particles from crossover hydrogen and oxygen, subsequent radical migration, and polymer attack. The response of the FER to new experimental conditions in this study suggests that the attack can exist at any plane within the membrane, not just the "Xo" plane of maximum Pt precipitation. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3095466] All rights reserved.