To clarify the essential features of Brill transition of aliphatic nylons, structural changes have been investigated by X-ray diffraction and infrared spectroscopic methods for the compounds CH3(CH2)10NHCO(CH2)9CH3 (designated as N11) and CH3(CH2)9NHCO(CH2)8CH3 (N10) which were assumed to be the simplest models of nylons 11/11 and 10/10, respectively. Depending on the preparation conditions, N11 crystallized to the α and γ forms and N10 to the α form at room temperature. Infrared and X-ray diffraction data clarified that the α form took essentially the same chain packing structure as that of the α form of parent nylons, in which the all-trans methylene segments were packed in a triclinic subcell and the amide groups were connected by intermolecular hydrogen bonds to form a sheet structure and these sheets were stacked by a weak van der Waals force. In the DSC thermograms, the α form of N10 and N11 showed two main endothermic peaks in the Brill transition (ca. 50 °C) and melting regions. The temperature re...