Branched sulfonated polymers exhibit excellent properties as proton exchange membranes (PEMs). However, very few highly branched sulfonated polymers are reported as PEMs. The highly branched polymer, including the method to increase degree of branching (DB) and the effects of DB on the properties of PEMs, should be further studied. In this work, novel branched sulfonated poly(fluorenyl ether ketone sulfone)s with different DB value are synthesized by direct polycondensation reactions from bisphenol fluorene (A(2)), sulfonated 4,4'-difluorobenzphenone, 1,3,5-tris(4-(4-fluorophenylsulfonyl) phenyl)benzene (B-3-3) and 4,4'-difluorodiphenyl sulfone. The highest DB with 10% branching agent is obtained using the B-3-3 monomer. The method to increase the DB is discussed. It is found that B-3 scaffold with long and hard arms can effectively increase the DB value. The effects of DB on the properties, including oxidative stability, proton conductivity, water uptake, swelling ratio, thermal stability, mechanical property and microstructure, are investigated. With increasing DB value, oxidative stability and proton conductivity of the membranes increase remarkably, but swelling ratio and tensile strength decrease slowly. The membrane with the highest DB value (10%) exhibits high proton conductivity (0.42 S cm(-1)) and oxidative stability (327 min), as well as relatively low swelling ratio (16.2%) at 80 degrees C. (C) 2014 Elsevier B.V. All rights reserved.