In order to clarify the differences between the in situ open circuit voltage (OCV) accelerated test and ex situ Fenton test, black polypyrrole (PPy) was specially selected to modify Nafion (R) membrane via in situ chemical oxidative polymerization. OCV decay rate, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) techniques are employed to characterize the membrane oxidation stability. It is found that the incorporation of PPy greatly enhances the membrane durability in the OCV test, while it makes no difference in Fenton test. And then, the differences between the OCV test and Fenton test are proposed in terms of the generation mechanism of free radicals, the amount of free radicals and the degradation mode. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.