Rotating ring-disk electrode (RRDE) testing is commonly used for determining the activity and selectivity of oxygen reduction reaction (ORR) catalysts. The effect of RRDE catalyst ink aging on the reported selectivity to water formation was studied for both commercial Pt/Vulcan carbon and nitrogen-containing nanostructured carbon (CNx) catalysts. Aging the ink before application could cause a significant improvement in the selectivity measured. While the reasons behind this improvement in selectivity are not yet clear, possible explanations include changes in physical properties of the catalyst, changes in surface hydrophilicity, and changes in surface oxygen functional groups. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3436658] All rights reserved.