Chemical stability of polymer electrolyte membranes (PEMs) is the key factor affecting the lifetime of fuel cells. It is greatly desirable to develop the PEMs with both high proton conductivity and excellent chemical stability. In this study, a series of sulfonated polyimide-polybenzimidazole copolymers (SPI-co-PBIs) are synthesized via random condensation polymerization of 1,4,5,8-naphthalene tetracarboxylic dianhydride, 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid, and an amine-terminated polybenzimidazole oligomer. The ion exchange capacities of the resulting SPI-co-PBIs are in the range 1.90-2.47 meq g(-1). Under fully hydrated condition, the SPI-co-PBI membranes show higher proton conductivities than Nafion112. It is found that the incorporation of a small fraction of PBI moiety into the polyimide structure resulted in significant improvement in radical oxidative stability. For example, the SPI-co-PBI-19/1 containing 5 mol % PBI moiety shows only 0.6 wt% weight loss after being soaked in the Fenton's reagent (3 % H2O2 + 3 ppm FeSO4) at 80 A degrees C for 150 min, whereas the corresponding benzimidazole group-free sulfonated polyimide is completely dissolved in the Fenton's reagent at 80 A degrees C for 140 min. The SPI-co-PBI membranes also show excellent hydrolytic stability due to the highly stable ladder structure of the benzimidazobenzisoquinolinone linkages.