A facile approach has been successfully developed for the preparation of a series of cross-linked sulfonated polyimide (SPI) membranes via the condensation reaction between the sulfonic acid groups and the activated hydrogen atoms of SPIs in the presence of phosphorous pentoxide : methanesulfonic acid in the ratio of 1 : 10 by weight (PPMA, method 1) or phosphorous pentoxide only (method 2). The resulting sulfonyl linkages are very stable and the cross-linked SPI membranes showed greatly improved water stability in comparison with the uncross-linked ones while high proton conductivity was maintained.