A series of pore-filling membranes are prepared by impregnating porous cross-linked benzoxazine-benzimidazole copolymer P(pBUa-co-BI) substrates with sulfonated poly(arylene ether sulfone)s (SPAES) s having different degree of sulfonation for polymer electrolyte membrane fuel cells operating at high-temperatures (> 100 degrees C) and low-humidity (< 50% RH) conditions. The SPAESs are synthesized by reacting 4,4'-dihydroxybiphenyl with the mixtures of disulfonate-4,4'-difluorodiphenylsulfone and 4,4'-difluorodiphenylsulfone in different ratios. The porous P(pBUa-co-BI) substrates are prepared by extracting dibutyl phthalate (DBP) included in P(pBUa-co-BI) films using methanol. The P(pBUa-co-BI) films are prepared by stepwise heating the casted N, N-dimethylacetamide solution containing the mixtures of poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI), 3-phenyl-3,4-dihydro- 6-tert-butyl-2H-1,3-benzoxazine (pBUa), and DBP to 220 degrees C. The pore-filling membranes are found to have much improved dimensional stability and mechanical strength compared with the SPAES membranes. Although the proton conductivity values of the pore-filling membranes are slightly smaller than those of the SPAES membrane, their cell performance is superior to that of the SPAES membrane at 120 degrees C and 40% RH conditions because ultrathin pore-filling membranes (15-20 mu m) having high mechanical strength can be prepared and they can contain a larger content of chemically-bound water.