Sulfonated poly(phthalazinone ether ketone) (SPPEK) membranes were cast from N,N-dimethylacetamide (DMAc) solution and tested for their application as proton exchange membranes (PEMs) in direct methanol fuel cells (DMFCs). The methanol and water swelling uptake and several permeation tests indicated that SPPEK had a lower affinity to methanol than Nafion and was less permeable than Nafion. In a single cell DMFC test at 70 °C with the membrane electrode assembly (MEA) made of SPPEK, the maximum power density was 55 mW/cm2 as the current density was 276 mA/cm2 and the ultimate (limiting) current density was 360 mA/cm2. The lower permeability of SPPEK compared with Nafion resulted in lower methanol crossover. Consequently, the optimal concentration of aqueous feed methanol (3 M) for the SPPEK MEA was higher than that of Nafion (2 M) under the same operation conditions. ; JNL0004966