A facile cross-linking method has been successfully developed for sulfonated polyimide (SPI) membranes. The SPIs with electron-rich phenyl rings in the non-sulfonated diamine moiety were readily cross-linked by immersing the membranes into a solution of phosphorous pentoxide and methanesulfonic acid at 30-80 degrees C. The resulting SPI membranes cross-linked via sulfonyl groups showed greatly improved water stability compared with the uncross-linked ones, while high proton conductivity was maintained. They have potential for fuel cells at high temperatures above 80 degrees C.