The differential sdlubility and thermal analysis of three kinds of polyester fibers designed for rotor, worsted ring, and carded ring spinning are measured. Poly(ethylene terephthalate) yarns are obtained through the three processes and then analyzed. Using a thermomechanical analyzer, the thermal transitions of the different yarn types are deter mined. The crystallinity of fibers and yarns is measured by DSC. The relationship between thermal transitions, crystallinity, and differential solubility of the polyester fibers and yarns is studied and some inferences on its fine structure are reported.