There is a growing literature investigating the effects of thermal annealing and green solvent exposure on the microstructural changes, in general, and transient structures in polyester fibers, in particular, using such techniques as birefringence, differential scanning calorimetry, X-ray diffraction, and dynamic mechanical analysis. The results obtained from these studies consistently indicate that transient structures in polyester fibers strongly depend on the medium used so that treatment in different media produces important changes such as the transformation of the intermediate domain into an amorphous or crystalline region. Compared to polyester samples annealed at high temperatures over long treatment times, those exposed to green solvents yield higher values of crystallinity, orientation, density, transient structure, crystallite size in lateral directions, and elastic modulus but lower values of fractional free volume. Being environmentally friendly, green solvents may, thus, be considered as superior replacements for the conventional materials used in thermal annealing, especially because they change the fine structure of polyester fibers over shorter treatment times and at lower temperatures.