The paper describes the preparation of membranes based on sulfonated poly(ether ether ketone) [SPEEK] [degree of sulfonation similar to 65%] in the presence of varying amounts of poly (ethylene glycol) (molecular weight 200) [PEG-200] and cyclohexane dimethanol [CDM] using water:ethanol (50:50) as solvent. After drying, the membranes were heat treated at 60 degrees C (2 h), 80 degrees C (2 h), 100 degrees C (2 h), 120 degrees C (2 h) and 135 degrees C for 16 h. After the heat treatment, samples were insoluble in water:ethanol (50:50) mixture. The membranes thus obtained were characterized by FTIR spectroscopy (structural), water uptake (hydrolytic stability), thermal stability (TG), mechanical stability and proton conductivity. A significant increase in the hydrolytic stability was observed, SPEEK became elastic and fragile whereas the heat-treated SPEEK/PEG and SPEEK/CDM remained stable even after 135 days of water immersion at 35 degrees C. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.