A series of sulfonated poly(ether ether ketone ketone)s (SPEEKK)s based membranes have been prepared and evaluated for proton exchange membranes (PEM). The membranes show very good thermal and mechanical stabilities. The structures of membranes were studied with AFM. The membranes show very good proton conductive ability (25 degrees C: 0.007-0.04 s/cm) and methanol resistance (25 degrees C: 7.68 x 10(-8) to 5.75 x 10(-7) cm(2)/s). The methanol diffusion coefficients of membranes are much lower than that of Nafion (2 x 10(-6) cm(2)/s). The SPEEKKs membranes show very good respective in direct methanol fuel cells (DMFC) usages. (c) 2005 Elsevier Ltd. All rights reserved.