This paper investigates the effects of methanol and water vapor on the performance of a high temperature proton exchange membrane fuel cell (HT-PEMFC). A H3PO4-doped polybenzimidazole (PBI) membrane electrode assembly (MEA), Celtec P2100 of 45 cm2 of active surface area from BASF was employed. A long-term durability test of around 1250 h was performed, in which the concentrations of methanol-water vapor mixture in the anode feed gas were varied. The fuel cell showed a continuous performance decay in the presence of vapor mixtures of methanol and water of 5% and 8% by volume in anode feed. Impedance measurements followed by equivalent circuit fitting revealed that the effects were most significant for intermediate-high frequency resistances, implying that charge transfer losses were the most significant losses. Vapor mixture of 3% in feed, however, when introduced after operation at 8%, showed positive or no effect on the cell's performance in these tests.