Organic sulfonated poly(ether ketone ketone) (SPEKK) membranes with different ion-exchange capacities (IECs), and composite membranes prepared by the addition of 20 wt % phosphotungstic acid (PTA) to SPEKK were used to prepare membrane electrode assemblies (MEAs). The proton conductivity of the membranes increased with increasing IEC of the SPEKK, and with the addition of PTA. The proton conductivity attained at 80 degrees C and 75% relative humidity was 20 +/- 2 mS/cm. The feasibility of using SPEKK in the cathode layer of the MEAs was investigated. The electrochemically active surface areas (ECAs) of the SPEKK-based cathodes were lower than that of the Nafion-based cathode and decreased further as the operating relative humidity was lowered. These observations were reflected in the single-cell polarization data, which indicated that the MEAs with the SPEKK-based electrodes were outperformed by their Nafion-based counterparts. Furthermore, a mismatch in SPEKK IEC between the membrane and cathode resulted in immiscibility at the interface. While the additive stability in the composite membrane was very good, the long-term stability of the membranes was poor when compared to perfluorosulfonic acid membranes such as Nafion, with failure occurring by scission along the gasket edges of the MEA after limited operation. (C) 2008 The Electrochemical Society.