The water stability of sulfonated copolyimides (SPIs) derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), sulfonated diamines of 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid (PBAPBDS), and 2,2'- or 3,3'-bis(3-sulfopropoxy)benzidine (2,2'- or 3,3'-BSPB) and nonsulfonated diamines was investigated in detail from viewpoints of viscosity, mechanical strength, proton conductivity, weight loss, and hydrolysis products eluted into the soaking water. With the aging in water or 100% relative humidity vapor at 130 degrees C, the polymer chain scission took place mainly in the early stage but thereafter slightly, and as a result the SPI membranes kept the reasonably high mechanical properties even after 192 h. The branched/cross-linked SPI membranes prepared by incorporating a flexible triamine kept better mechanical properties. The weight loss and sulfur loss of < 10% were observed for the pBAPBDS-based SPIs. This was due mainly to the elution of hydrolysis product, the oligomer of NTDA, and sulfonated diamine, which did not contain the nonsulfonated diamine moieties. With the aging at 130 degrees C, the proton conductivity did not change for the pBAPBDS-based SPIs, but for the BSPB-based SPIs it decreased 20% in water and much more at the lower relative humidities because of the cleavage of the sulfopropoxy group. The accelerated water stability tests reveal that the water stability of the present SPI membranes is not sufficiently high at 130 degrees C but is high enough for PEFC and DMFC applications at least at 80 degrees C.