A novel alternating copolymer of sulfonated poly(ether ether ketone-alt-benzimidazole)s (SPEEK-alt-BI) was synthesized by polycondensation in our previous report. In this article, we examined the properties of SPEEK-alt-BI membranes, such as thermal and mechanical stability, hydrolysis, and swelling properties in water, and proton conductivities at various temperatures and humidity. The experimental results showed that SPEEK-alt-BI membranes exhibited promising oxidative and hydrolysis stabilities. Although the base moieties in SPEEK-alt-BI main chain would capture proton and form reversible salt hindering the proton transfer capability, the conductivity of membrane 60SPEEK-alt-BI-H [Sulfonation degree (S.D.), 60% in form of proton] could achieve 22.61 mS/cm at 80 degrees C and 100% humidity. Meanwhile, the conductivity of membrane could reach 40.12 mS/cm with phosphorus acid doping. The methanol permeation coefficient of 60SPEEK-alt-BI membrane is 2.3 x 10(-8) cm2/s at 20 degrees C, which is magnitude lower than that of Nafion (R) 117, 1.11 x 10(-6) cm2/s. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012