A two-beam "Interphako" interference microscope was used to study the effect of annealing on the physical properties of high-tenacity poly(ethylene terephthalate) H-T PET fibers. The PET fibers were annealed with free ends for I h at temperatures ranging from 100 to 200 degrees C. The shrinkage, refractive indices, and orientation angle of the PET fibers were determined for different annealing temperatures. The measured birefringence and orientation function were found to have decreased with increasing temperature, whereas the degree of crystallinity and the onset temperature (DSC) increased. POLYM. ENG. SCI., 48:2329-2334, 2008. (C) 2008 Society of Plastics Engineers