The crystalline phase of polyamide 6 (otherwise nylon 6) films produced following various physical treatments is investigated by means of thermal analysis, X-ray diffraction, and infrared spectroscopy. A recently published procedure for treating infrared spectra of multicomponent compounds without a priori knowledge of the individual component spectra allowed us to perform a semiquantitative analysis of the structural changes upon annealing, including data from drawn samples. Melt-cast films display a mesomorphic state that is thermally stable up to about 120 degreesC. This structure partly reorganizes into the stable monoclinic a form above 120 degreesC. Films in major gamma form produced by iodine treatment are thermally stable up to 200 degreesC. Films in major alpha form are also prepared by superheated water treatment. No evidence is given for a Brill transition about 170C. This is an important fact for further understanding of the drawing behavior of PA6 at temperatures above and below this domain. The mesomorphic phase can hardly be separated from the amorphous component both from X-ray and infrared analysis. However, scanning calorimetry, which is often criticized due to possible reorganization of unstable species during the heating scan, turned out to be a very useful technique. Indeed, recrystallization from the amorphous phase or improvement of ordering from the mesomorphic state both result in exothermic effects in quite different temperature domains that allow to discriminate the two phenomena. (C) 2001 John Wiley & Sons, Inc.