The lamellar structures in nylon 6,6 were characterized at several temperatures using synchrotron small-angle X-ray scattering (SAXS) to monitor the changes during the Brill transition at similar to 160 degrees C (T-B) The integrated intensity and the coherence length of the lamellar stacks increase during heating above T-B. In contrast, during cooling, the intensity increases below T-B whereas the coherence length increases up to T-B and remains unchanged upon further cooling. The increase in SAXS intensity observed above T-B during heating is attributed to the enhanced contrast resulting from a decrease in the packing density of the amorphous chain segments. It is suggested that the changes in the packing of the crystalline stems, as they do occur during crystalline phase transitions, provoke a proportionate response in the packing of the amorphous chain segments in the interlamellar regions. The thermal expansion coefficient of the amorphous segments calculated from the X-ray diffraction data above T-B is 5.9 x 10(-4)/K (molar thermal expansion coefficient, 854 x 10(-10) m(3)/(mol K)), and this is in reasonable agreement with the published values.