Polystyrene -ethylene-butylene sulfonate copolymer membranes (PSEBS) were photochemically cross-linked using benzophenone as the photo-initiator. Benzophenone decomposed by first-order kinetics during the cross-linking process. The cross-linked PSEBS had lower water swelling, lower proton conductivity, and higher chemical stability than un-cross-linked PSEBS. The cross-linking process did not change the ion exchange capacity (IEC); the sulfonic acid groups were stable during the UV photo-cross-linking process. The photo-cross-linked PSEBS membranes degraded more slowly during fuel cell operation than non-cross-linked membranes. A layered membrane with PSEBS sandwiched between thin Nafion layers degraded at almost the same rate as PSEBS membranes. Nafion layer contacted with electrodes cannot prevent membrane from degradation during the fuel cell operation; degradation reaction carried out all through the membrane between cathode and anode.