The properties of sulfonated poly(ether ether ketone) (SPEEK) were investigated by various solvents such as N-methly-2-pyrrolidone (NMP), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and dimethylformamide (DMF). The SPEEK membranes were activated by the solution of 1 M H2SO4 to remove the remaining solvents in the membranes. The properties of proton conductivity and cell performance of the membranes after activated treatment were improved compared to those of before activated treatment. Especially, the proton conductivity and cell performance of the SPEEK membrane using NMP as a solvent was the equal to that of the Nafion 117 membrane. We suggest that the selection of a casting solvent and membrane activation treatment for removing of the remaining solvent in the membrane can be a very important factor to get a high performance membrane. (C) 2011 Elsevier B. V. All rights reserved.