Novel sulfonated poly(ether ether ketone ketones) (SPEEKKs) based membranes have been prepared and evaluated for proton exchange membranes (PEMs). Directly aromatic and nucleophilic substituting polycondensation was adapted to control the sulfonated process. SPEEKKs with different sulfonated degrees were prepared by adjusting the content of sulfonated monomer. All the SPEEKKs can easily be cast into tough membranes and show high thermal stability. SPEEKKs membranes exhibit conductivities (25° C) from 3.6 x 10(-4) to 0.05 S/cm, (80° C) from 0.001 to 0.06 S/cm; water swelling from 9.14 to 26.71%, ion-exchange capacities (IEC) from 0.63 to 1.57 mmol/g and methanol diffusion coefficients at 25 ° C from 6.6 x 10(-7) to 8.6 x 10(-7) cm(2)/s. These diffusing coefficients are much lower than that of Nafion® (2 x 10(-6) cm(2)/s) and make SPEEKK membranes with a good alternative that can reduce problems which associates with high methanol crossover in direct methanol fuel cells. © 2005 Elsevier B.V. All rights reserved.