The degradation of two commercial (E-TEK) electrocatalysts, made of palladium nanoparticles supported on Vulcan XC-72 (Pd/C) and differing by their average Pd nanoparticles diameter, was studied in alkaline and acidic solutions. The fate of the electrocatalysts upon accelerated stress test (AST) in 0.1 M NaOH was compared to that in 0.1 M H2SO4. Whereas Pd/C nanoparticles suffer dramatic degradation in 0.1 M H2SO4, electrochemical measurements combined with Identical-Location Transmission Electron Microscopy (ILTEM) demonstrate their higher stability in 0.1 M NaOH at 25 degrees C than reported in the literature for Pt/C electrocatalysts (supported on the same support). As palladium is an efficient anodic material for direct alkaline fuel cells, the stability of Pd/C was investigated in the presence of reducing species in the alkaline electrolyte: alkaline electrolytes containing hydrogen or hydrazine borane yield slightly larger instability for the Pd/C nanoparticles, and the effect is less pronounced for large Pd/C nanoparticles than for small ones. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.