Membrane electrode assemblies (MEAs) made from polyetheretherketone (PEEK) showed excellent fuel cell performance and thermal stability in the presence of substantial CO at elevated temperatures (i.e., 120degreesC) in proton exchange membrane fuel cells (PEMFCs). For example, the current from a MEA made from PEEK membrane at 0.6 V and 120degreesC was 0.50 A/cm(2) when run on pure hydrogen and 0.45 A/cm(2) when run on reformate (50% H-2, 1300 ppm CO, and balance N-2). The current density from a MEA made from Nafion at 0.6 V and 120degreesC was 0.61 A/cm(2) when run on pure hydrogen. The main difference between these two MEAs is that the ionic conductivity of the PEEK membrane at 120degreesC was 3.38x10(-2) S/cm, which is approximately three times lower than Nafion. Although it is not surprising that CO tolerance increases with increasing temperature, we are the first to show less than 10% drop in performance with 1300 ppm CO at 120degreesC. Even though Nafion membranes have higher conductivity, PEEK membranes lasted for 350 h thereby outlasting Nafion membranes by seven to eight times. (C) 2003 The Electrochemical Society.