Sulfonated poly(ether ether ketone) (SPEEK) with different degrees of sulfonation has been prepared and evaluated as proton exchange membrane electrolytes in direct methanol fuel cells (DMFCs). The membranes have been characterized by ion-exchange capacity, proton conductivity, and liquid uptake measurements. The proton conductivity of the SPEEK membranes increases with increasing sulfonation level, and are lower than that of Nafion. The percent liquid uptake increases with increasing temperature, methanol concentration, and degree of sulfonation. Within a narrow range of sulfonation of similar to50%, the SPEEK membranes exhibit electrochemical performances comparable to or exceeding that of Nafion at 65degreesC, making it an attractive low-cost alternative to Nafion. The better performance of the SPEEK membranes is due to the suppression of methanol permeability as indicated by a lower methanol crossover current density at the cathode. (C) 2003 The Electrochemical Society.