This study focuses on investigating the performance change of a high temperature proton exchange membrane fuel cell (HT-PEMFCs) stack at different operation modes. A HT-PEMFC stack consisting of 30 single cells was tested both at constant load (0.4 A cm−2) and dynamic load (0.05–0.4 A cm−2) conditions at a temperature of 160 ℃ and hydrogen as anode fuel. Besides, the effect of impurities on the stack was also investigated by feeding a methanol reformate mixture to the stack anode as fuel for both constant and dynamic operation. The results reveal that the stack performance was stable after 120 h of both constant and dynamic operation with hydrogen, while the stack performance decreased greatly when the stack was fed with dry reformate on the anode. Significant degradation rates of 94.4 µV h−1 for constant operation, while the degradation was 200 times higher in dynamic operation with reformate gas.