Details on the degradation mechanism of hydrocarbon ionomers for polymer electrolyte membrane fuel cells (PEFCs) have been investigated using density functional theory (DFT). Two model compounds of a hydrocarbon ionomer were selected to investigate the nature of the degradation, where the compounds selected represented the general features of available hydrocarbon polymer electrolyte membranes (PEMs). The results show that two degradation reactions are energetically favorable, indicating two possible weak sites in the hydrocarbon PEMs susceptible to OH or H radical attack. One site has an aryl sulfonated bond and a sulfonate group, and the other site has an aryl ether bond. The alkane chains in the PEMs were calculated to be relatively stable against radical attack. In addition, we found that in contrast to the degradation of perfluorinated PEMs, hydrocarbon PEMs are relatively robust against H radical attack. (C) 2015 Elsevier B.V. All rights reserved.