Ultraviolet microspectrophotometry is used to monitor the radial diffusion of three uv stabilizers in high molecular weight PET fibers. Under identical application con ditions, the benzotriazole and benzotriazine derivative stabilizers show peripheral penetration only, whereas the benzophenone derivative stabilizer diffuses throughout the fiber cross section. Peripheral penetration of the benzotriazole and benzotriazine derivatives does not prevent polymer photodegradation in the unprotected interior, whereas all regions of the fiber are protected by the more uniformly distributed ben zophenone stabilizer. Heavy peripheral deposition of the benzotriazole stabilizer pre vents these highly protected regions from polymer photodegradation.