Part I of this study carried out membrane electrode assembly degradation of a four-cell stack with Nafion membranes of different thicknesses, including N117, N115, NR212, and NR211, for 1000 h under idle conditions. Through on-line electrochemical measurements it was found that as degradation advanced, cells with thinner membranes experienced much more rapid performance degradation than those with thicker membranes, especially after 800 h of operation, due to a dramatic increase in hydrogen crossover. In the present work we investigate the degradation mechanisms of this four-cell stack using several ex situ diagnostic tools, including scanning electron microscopy (SEM), infrared (IR) imaging, ion chromatography (IC), gas permeability measurement, contact angle measurement, and simulation. The results indicate that the drastic increase in hydrogen crossover is due to membrane thickness loss and pinhole formation. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.