The membrane durability in proton-exchange membrane fuel cells (PEMFCs) is the major obstacle limiting its applications, especially in vehicles. Radical attack reactions are thought to be the main reason for degradation in PEMFCs and these radicals are generated during the fuel cell operation. However, radical attack mechanisms are still incomplete and there are contradictions between experimental result and theoretical prediction. For example, ab initio modeling studies all indicated that the hydrogen radical H· can easily attack to the oxygen atoms on the sulfonic anion, leading to the degradation product of HSO₃^-. However, little sulfur contents were observed experimentally. Based on previous experimental evidence on isotopic substitution, recently we had a hypothesis that the hydronium radical (H₃O·) might be present in PEMFCs by the reaction between H· and a water molecule. We've performed ab initio modeling for this radical in the PEM environment. We found that this radical can be greatly stabilized by anions on the polymer side chain but destabilized by the proton. When the side chain undergoes a conformational change to take the H₃O· radical that associated with the sulfonic anion closer to the polymer backbone, greatly reduced reaction barriers for degradation reactions on or near the backbone (tertiary F or ether O) could be observed. Thus, our H₃O· hypothesis is able to explain not only the previous isotopic substitution experiment, but also explain why H· doesn't attack the sulfonic anion. We believe this hypothesis would lead to new insights to design PEM with higher durability.