A novel sulfonated diamine monomer, 3-(2',4'-diaminophenoxy)propane sulfonic acid (DAPPS), was successfully synthesized and the sulfonated polyimide (SPI) was prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and DAPPS. The resulting SPI, NTDA-DAPPS, was soluble in common organic solvents. The SPI membrane displayed proton conductivity sigma values of 0.12-0.35 S/cm at temperatures ranging from 35 to 90 degreesC in liquid water, which were similar to or higher than those of Nation 117 and sulfonated hydrocarbon polymers. The sigma of the SPI membrane decreased significantly with decreasing relative humidity (RH) and became much lower than that of Nation 117 at 30% RH. The SPI membrane displayed good water stability at 80 degreesC and was thermally stable up to 240 degreesC. It showed reasonable mechanical strength of a modulus of 1.3 GPa at 90 degreesC and 90% RH. Its methanol permeability PM was 0.57 x 10(-6) cm(2)/s at 30 degreesC and 8.6 wt% methanol in feed, which was a fourth of that of Nation 117. As a result, its ratio of sigma/P-M was 21 x 10(4) S cm(-3) S, which was about 4 times larger than that of Nafion 117, suggesting potential application of the SPI membrane for direct methanol fuel cell. (C) 2003 Elsevier Science Ltd. All rights reserved.