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)(10)NHCO(CH2)(9)CH3 (designated as N11) and CH3(CH2)(9)NHCO(CH2)(8)CH3 (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 (x and gamma forms and N10 to the alpha form at room temperature. Infrared and X-ray diffraction data clarified that the (x form took essentially the same chain packing structure as that of the (x form of parent nylons, in which the all-trans methylene segments were packed in a triclinic subcell and the amide groups were connected by inter-molecular hydrogen bonds to form a sheet structure and these sheets were stacked by a weak van der Waals force. In the DSC thermograms, the alpha form of N10 and N11 showed two main endothermic peaks in the Brill transition (ca. 50 degreesC and melting regions. The temperature region was divided into three (1, 11, and 111). In region I below 50 degreesC, the crystal was of the alpha type. In region III (above 58 degreesC for N10 and above 65 degreesC for N11), the crystal transferred to the high-temperature gamma form (gamma(h)), in which the molecular chains had disordered conformation, as speculated from the change in methylene progression bands and the broad amide bands, and these chains were packed in a pseudohexagonal mode. During the transition from region I to 111, another phase was observed to appear discontinuously in region II. This intermediate phase showed the infrared spectra composed of the characteristic patterns of the (x and gamma(h) forms but the X-ray diffraction pattern was quite unique and different from those of the latter two forms, indicating that the molecules with a hybrid conformation between the alpha and gamma(h) forms were packed in a unique crystal lattice. In case of the gamma form of N11 compound, the molecular chains took the conformation built by skewed amide groups and partially disordered trans methylene segments even at room temperature. The crystal transferred apparently continuously to the gamma(h) form above 70 degreesC.