A crystalline condensate of dibutyltin oxide and tributyl phosphate (Sn-P-c) was more effective than the stannous octoate and dibutyltin dilaurate in accelerating the curing of the epoxy resin/4,4'-diamino diphenyl sulfone (DDS) system, based on an equal tin concentration. Water took part in the reaction and played an important role in the morphology and mechanical properties of the cured resin when Sn-P-c was used as the accelerator. In the presence of Sn-P-c, water first reacted with glycidyl ether to yield glycerol ether; the yielded hydroxyl group may further catalyze the amino/epoxy group reaction. In the absence of water or polar hydroxyl additives, Sn-P-c did not completely dissolve in the resin systems, and the cured resin was translucent material with poor mechanical strength. In the presence of a small amount of water, the cured material became transparent. Both flexural strength and maximum deflection were increased greatly.