Acidic polymer electrolyte membranes (PEM) based on sulfonated poly(ether ketone ketone) (SPEKK) with relatively moderate to high levels of sulfonation (ion-exchange capacity (IEC) > 1.7 meq/g) have excellent proton conductivities (similar to 0.1 S/cm), but they absorb excessive amounts of water at elevated temperatures (ca. 60-90 degrees C). To reduce the water sorption of the membranes and improve their mechanical properties and dimensional stability, the protons in acidic SPEKK membranes were partially exchanged with divalent barium cations to create ionic crosslinks between the sulfonate groups attached to the aromatic rings of the PEKK. The degree of crosslinking was varied by changing the degree of neutralization of the ionomer. The proton conductivity, water uptake, and methanol permeability at various levels of sulfonation and degree of crosslinking were measured. The conductivity was reduced by crosslinking, but the thermal stability, swelling, and barrier properties were improved. Crosslinking reduced the water swelling at room temperature by nearly a factor of two and prevented membrane dissolution at temperatures up to 80 degrees C. In a water/methanol mixture (72/28, v/v), swelling was reduced by a factor of four. The balance of transport and mechanical properties could be varied to produce a viable PEM for a direct methanol fuel cell by adjusting the crosslink density. (c) 2007 Elsevier B.V. All rights reserved.