The crystallization of polyamide 6.12 was investigated by X-ray diffraction and infrared spectroscopy. Similar to polyamide 6.6, there exist a pseudohexagonal γ modification and a triclinic α modification. The characteristic IR absorption bands of each modification were identified and assigned. In the amorphous state, polyamide 6.12 additionally shows an absorption band at 1255 cm−l. The γ modification can be obtained by heating an amorphous sample above the glass transition temperature near 40°C and is stable only at high temperatures. Cooling of the γ modification immediately resultsinthe Brill transition into the α modification. It could be shown that the Brill-transition temperature is identical with the annealing or crystallization temperature of the Y modification. The difference of the 100 and 010 lattice plane distances measured at room temperature as well as the ratio of the IR absorbances between some a and y bands increase with increasing annealing temperature and can be used for the determination of the individual Brill-transition temperature of a sample.