A series of cross-linked sulfonated poly(ether ether ketone) (SPEEK) membranes were synthesised using six different molecular weight of poly ethylene glycol (PEG) [200, 400, 600, 3000, 6000, 10,000 Da] as cross-linkers to investigate the effect of cross-linker chain length on membrane morphology and properties. Membranes morphology were investigated by using atomic force microscopy (AFM) and small angle X-ray scattering (SAXS), which showed that there was formation of cluster like structure and hydrophilic-hydrophobic phase separation occurs after cross-linking that makes the membranes mechanically stronger and reduces its swelling in water at higher temperature. Membrane properties were found to be strongly dependent on the chain length of the cross linkers. PEG-400 cross-linked membrane gave desirable properties in terms of overall membrane performance such as proton conductivity, mechanical strength, and membrane durability etc. The methanol permeability of the membrane is found to be lower than Nafion-117 up to 55 degrees C.