A series of sulfonated polyimides (SPIs) containing pyridine ring in the polymer backbone were synthesized by the polycondensation of 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTDA), 5-(2,6-bis(4-arninophenyl)pyridin-4-yl)-2-methoxy benzene sulfonic acid (SDAM), and 4,4'-diaminodiphenyl ether (ODA). Flexible, transparent, and tough membranes were obtained. Property study revealed that all the membranes displayed high thermal stability with the desulfonation and decomposition temperature higher than 290 and 540 degrees C, respectively, as well as good mechanical property with Young's modulus larger than 1.0 GPa, maximum strength (MS) on a scale of 60-80 MPa, and elongation at break (EB) ranged from 41.79 to 75.17%. More importantly, the new materials exhibited small water uptake and excellent dimensional stability with the highest sulfonated SPI-80 showing the maximum water uptake of 36.1%, and maximum swollen ratio of Delta t = 0.038 and Delta l = 0.026, respectively (Delta t and Delta l stands for the thickness and diameter change of the film, respectively). The high water stability exhibited by the SPI films is attributed to the formation of inner salts and/or ionic crosslinking between the sulfonic acid and pyridine functional groups, which suppresses the water uptake ability of sulfonic acid and strengthened the interpolymer chain interactions. Thus, the excellent water stability, good thermal and mechanical properties, and the technologically applicable conductivity of SPI-80 render this material attractive for proton exchange membrane (PEM) application. (C) 2009 Wiley Periodicals, Inc. J Appl Polyrn Sci 114: 3190-3197, 2009