Abstract: Sulfonated poly(ether ether ketone) (SPEEK), with excellent mechanical properties and chemical stability, is one of the promising polyelectrolyte membranes (PEMs) for fuel cells applications. However, the dimensional stability of SPEEK is reduced at certain degree of sulfonation (DS > 70%); and at operation temperatures above 60 ^°C. One of the practical approaches for improving membrane integrity and dimensional stability is through formation of covalent crosslinks between SPEEK polymer chains. In this article, a simple and novel approach is introduced for enhancing the dimensional stability of SPEEK membranes through hypercrosslinking methodology. SPEEK membranes were hypercrosslinked with formaldehyde and the resulted membranes exhibited reduced water swelling and methanol permeability with negligible effect on the original proton conductivity.