Ex situ hydrolytic stability of sulfonated polyimide (s-PI) membranes for fuel cells was studied depending on structural and external parameters including the ion exchange capacity, the block character, the temperature and the hydrogen peroxide concentration. Infrared spectroscopy was used to identify and quantify the chemical modifications such as the loss of imide functions and of ionic monomers. The decrease in ion exchange capacity due to the elution of sulfonated oligomers was confirmed by sulfur content analysis. A complete hydrolysis of some of the imide functions is observed leading to polymer chain scissions and to the loss of the mechanical properties. It is shown to be a thermo activated process and the activation energy (60 kJ/mol) is found in good agreement with the value determined from fuel cell lifetimes. The degradation in fuel cell conditions is similar but faster than in pure water. The same kinetic can be reproduced ex situ by addition 0.05% of hydrogen peroxide. (c) 2006 Published by Elsevier Ltd.