A novel method for the crosslinking of poly(ethylene terephthalate) fibers is described using 1,6-hexanedisulfonyl azide, 1,3-benzenedisulfonyl azide, and 2,6-naphthalenedisulfonyl azide. The azides are diffused into poly(ethylene terephthalate) fibers (Dacron) from perchloroethylene solution, and the fibers are heat treated to bring about decomposition of the sulfonyl azide and give rise to crosslinking. A study is made of the mechanical and thermal properties of the resultant fibers, which are changed considerably in comparison to the untreated fiber.