We successfully prepared membrane electrode assemblies (MEAs) for polymer electrolyte fuel cells using aligned carbon nanotubes (ACNTs) as the cathode catalyst support and evaluated their performances in single-cell tests. The ACNT-based MEAs demonstrated improved performance over a commercial carbon-black-based reference MEA in voltage-current polarization studies, particularly in the high current, mass-transport-limited region. The ACNT-MEAs also showed a significantly higher stability in conductivity and corrosion resistance than that of the reference during an accelerated aging test through potential cycling, which can be attributed to the vertical alignment and high graphiticity of the carbon nanotubes. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3360901] All rights reserved.