A 36-cell proton exchange membrane fuel cell (PEMFC) stack was contaminated with 50 ppm propene in air. Propene contamination amplified the uneven cell performance distribution along the stack length. End cells showed a larger performance change due to contamination than contiguous cells owing to a lower temperature and a larger effect of contamination at lower temperatures. The performance change of the inner cells linearly varied from cell 2 to cell 35 and was attributed to several causes including the uneven sub-saturated air flow distribution and the propene oxidation reaction involving a water molecule. The inner cells performance distribution was also credited to the uneven coolant flow distribution and a large effect of temperature on contamination. Higher cathode potentials acted as a cleaning method that minimized the contamination effect by promoting propene oxidation and led to weakly adsorbing CO2. As a consequence, higher cathode potentials also resulted in smoothing the uneven inner cells performance distribution.