Formation of sulfonic anhydride S-O-S (from condensation of sulfonic acids) is known as a possible degradation mechanism for Nafion® membranes under hydrothermal aging condition, which is especially critical for hydrogen fuel cells. A similar mechanism may also contribute to the membrane degradation in direct methanol fuel cells (DMFCs), where liquid water has direct contact with the electrolyte. An ex-situ experiment was established with swelling-dehydration cycles on the membrane. However, formation of sulfonic anhydride was not detected during the entire treatment; instead contamination from traces calcium in the nominally pure water used in the experiment was found to be the primary reason for the deterioration of the membrane properties. Trace impurities in the liquid methanol feed in DMFC may therefore represent an important contamination source.