Sulfonated poly(ether ether ketone) containing hydroxyl groups (SPEEK-OH) has been prepared for use as a proton exchange membrane (PEM) by reducing the carbonyl groups on the main chain of the polymers. With the goal of reducing water uptake and methanol permeability, a facile thermal-cross-linking process is used to obtain the cross-linked membranes. The properties of the cross-linked membranes with different cross-linked density are measured and compared with the pristine membrane. Notably, SPEEK-4 with the highest cross-linked density shows a water uptake of 39% and a methanol permeability of 2.52 x 10(-7) cm(2) s(-1), which are much lower than those of the pristine membrane (63.2% and 5.37 x 10(-7) cm(2) s(-1), respectively). These results indicate that this simple approach is very effective to prepare cross-linked proton exchange membranes for reducing water uptake and methanol permeability. (C) 2010 Elsevier B.V. All rights reserved.