Textile fibers are usually characterized by observing their morphology with an optical microscope, by qualitatively identifying the polymer composition by Fourier Transform Infrared (FTIR) spectroscopy, and by a microspectrophotometric comparison of color if the items are dyed. Recently, a method has been proposed to differentiate morphologically similar colorless PET single fibers, using FTIR microspectroscopy. In the current study, 10 colorless PET commercial samples were selected. All the fibers matched after a comparison of the morphology by optical microscopy. The samples were subjected to conditions that simulated exposure to sunlight and immersion in water. This report describes the procedures used with infrared spectroscopy, differential scanning calorimetry, and optical microscopy. The procedure found that the ratios between infrared bands at 3440cm-1 and at 874 cm-1 (A3440/A874) and between signals at 1370cm-1 and 846cm-1 (A1370/A846) were unaffected by long exposures (3 months) to strong sunlight or immersion in water. The fibers remained morphologically and structurally unaltered. This study thus validates the significance of comparisons between fibers kept in different environments. 1 table, 5 figures, and 34 references