Nafion(R) is an important polymer electrolyte for polymer electrolyte fuel cell applications due to its inertness and high proton conductivity. Operation of these fuel cells for extended periods of time at temperatures approaching 100 degrees C introduces concerns of the thermal stability of the Nafion electrolyte. To simulate the conditions in a fuel cell, Nafion samples were loaded with fuel-cell grade platinum black and heated under atmospheres of nitrogen, 5% hydrogen, or air in a thermal gravimetric analyzer. The products of decomposition were taken directly into a mass spectrometer for identification. In all cases, Nafion was found to be thermally stable up to 280 degrees C, at which temperature the sulfonic acid groups began to decompose. A mechanism for the decomposition is proposed which explains many of the evolved compounds observed during heating.