The electrochemical degradation of commercial (E-TEK) Pt and Pd nanoparticles supported on Vulcan XC-72 (noted Pt/C and Pd/C) was performed in NaOH electrolyte solutions. The fate of the electrocatalysts upon accelerated stress test (AST) in alkaline medium was compared to that in acidic (H2SO4 and HClO4) electrolyte solutions. Electrochemical measurements, combined with Identical-Location Transmission Electron Microscopy (ILTEM) demonstrated that the Pt/C catalyst degradation in 0.1 M NaOH at 25°C is extremely severe for a mild accelerated stress test (AST \textendash 150 cycles, at 100 mV sec-1 and 25°C, between 0.1 and 1.23 V vs. RHE), mainly owing to large nanoparticles detachment from the carbon substrate. For Pd/C, an opposite trend is found: the degradations are harsher in acidic environment.