Temperature-induced crystalline phase transitions in neat nylon 6 fibers as well as nylon 6/montmorillonite nanocomposite fibers have been studied by means of wide-angle X-ray scattering. Both types of melt spun fibers only consist of the y crystalline phase that does not display any transition during heating up to the melt. In contrast, fibers drawn up to the maximum draw ratio at 140 degrees C display the single a phase with a high degree of chain orientation. During the temperature increase, the a phase undergoes a gradual structural disordering but preserves its monoclinic character up to melting. The structural evolution of the a form turned out sensitive to the thermal and mechanical treatment of the fibers. Annealing the unfilled drawn fibers at 150 degrees C prior to the WAXS experiment improves the thermal stability of the alpha form due to heating of the processing-induced crystalline defects. The montmorillonite-filled fibers display both the a and the y crystals, which readily turn into a crystal form only upon drawing. Due to the matrix shearing between the MMT platelets, the H-bonded sheets display a higher thermal stability as compared with unfilled drawn fibers. Upon cooling from the melt, the first signs of crystallization are of y form in the MMT-PA6 fibers, but the a form rapidly turns predominant. Crystallization kinetics considerations are put forward to account for this finding. (c) 2006 Elsevier Ltd. All rights reserved.