Sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) for proton exchange membranes were prepared by sulfonation of PPESK using fuming acid and sulfuric acid agent. The separation of SPPESK was carried out by precipitating in ether and then in acetone. The degree of sulfonation (DS) was determined by ion exchange or conductimetric titration method. The FTIR study and H-1 NMR analysis were used to confirm the SPPESK has been synthesized successfully. Membranes were cast from SPPESK solution in NMP. Thermal stability, water uptake and swelling rate, proton conductivity and methanol permeability of SPPESK membranes were studied. SPPESK have excellent thermal stability (DS = 28-132%; T-d: 300-305 degrees C; Tg: 287-301 degrees C), appropriate water uptake (<= 78%) and swelling rate (<= 37%) when DS <= 81%. The conductivity of SPPESK membranes increases with DS and temperature. The conductivity of SPPESK 81% and SPPESK 91% reach 1.3 and 3.0 x 10(-2) S/cm at 80 degrees C, near to that of Nation115 (3.1 x 10(-2) S/cm) at the same conditions. The apparent activation energies of the conductivity Delta E-a of membranes with SPPESK 81% and SPPESK 91% are 20.5 and 22.20/mol, respectively. Those values are higher than that of Nafion115 (10.8kJ/mol). SPPESK membranes exhibit methanol permeability at 15 degrees C from 1.3 x 10(-7) to 9.7 x 10(-8) cm(2)/s depending on the DS from 0 to 99%, those values are reduced by a factor of 32-42 for that of Nation 115 membrane (4.2 x 10(-6) cm(2)/s) at the same conditions. All the above properties of SPPESK (especially DS 60-80%) indicate this material is promising for possible use in proton exchange membrane fuel cells, especially in direct methanol fuel cells. (c) 2006 Elsevier B.V. All rights reserved.