This study proposes an in situ accelerated stress test of a gas diffusion layer (GDL) at a gas-solution-electrode triple phase boundary to individually examine electrochemical and mechanical GDL aging for the first time. Electrochemical GDL stability during repeated potential jumps and mechanical GDL robustness during inert gas permeation were investigated. A Pt-loaded GDL was used to mimic a GDL in contact with Pt particles at the cathode. It was also used to evaluate GDL degradation during an accelerated stress test. In this study, the GDL that experienced an electrochemical stress of potential jumps up to 1.75 V for 27.8 h exhibited 2.9-fold and 4-fold higher losses in electrochemical surface area and oxygen reduction current, respectively, than did one eroded by Ar permeation at 325 cm3 min-1 for 100 h.