The crystallization of nylons 6,10, 6,12, and 4,6 was monitored in situ by X-ray diffraction from the melt. Nylons 6,10 and 6,12 were found to crystallize into the alpha-phase as indicated by the two-peak nature of the diffractogram. Nylon 4,6 crystallized with a single peak, which is characteristic of the gamma-phase (hexagonal structure). On cooling from the crystallization temperature to room temperature, nylons 6,10 and 6,12 preserved the two-peak nature; however, detailed data analyses indicated a change from the high-temperature (HT) alpha-phase to the low-temperature alpha-phase (triclinic structure) below 200 degrees C. Amorphous nylons 6,10 and 6,12 crystallize in the hexagonal structure when heated from room temperature (RT) and transform into the HT alpha-phase above 190 degrees C. But if cooled before this transition, these samples exhibit gamma-phase to alpha-phase transitions (Brill transitions). Nylon 4,6 crystallized from the melt in the gamma-phase, transformed into the HT alpha-phase about 40 degrees C below the crystallization temperature and showed a further transition similar to 150 degrees C below the crystallization temperature. The second transition is from the HT alpha-phase to the RT alpha-phase.