AbstractA variety of annealing treatments at elevated temperatures have been characterized by a specific infrared absorption assigned to the folded chain segments in polyethylene terephthalate fibers. The higher the annealing temperature, the more regular is the chain folding introduced into the fiber. When a fiber is free to contract, it will have a much larger amount of refolding than one which is held at constant length. Stretching is able to prevent refolding at low temperatures but in these experiments stretching was not able to prevent refolding at the highest temperature.By comparing the above infrared results with small angle X‐ray diffraction and broadline nuclear magnetic resonance techniques, it is now possible to give validity to the interpretation that chain folds are major contributors to the long period.The external tension applied to a fiber during an annealing treatment will determine the internal shrinkage tension caused by the molecules during subsequent reheating. If chain refolding is allowed to occur during annealing, the potential for subsequent shrinkage and the amount of internal shrinkage tension are lowered, and higher temperatures are required to activate additional shrinkage. It thus appears that a major contributor to shrinkage and shrinkage tension in an oriented fiber is the refolding of molecules during thermal activatio