Crystallization in polyamide 6 (nylon 6) fibers during annealing was studied in detail by following the changes that occurred in the neighborhood of crystalline relaxation temperatures, by using wide- and small-angle X-ray scattering and differential scanning calorimetry (DSC). Two distinct crystallization regimes were observed depending on whether annealing was carried out below or above onset of crystalline relaxation at similar to 1.90 degrees C. In fibers annealed below 190 degrees C, minor melting peaks were followed by exothermic transitions. These were attributed to similar to 1.5% (by weight) of microcrystals formed during annealing that melt and recrystallize during the DSC scan. These microcrystals are nucleated from unoriented amorphous chains between the lamellar stack within a fibril, and are shown to account for the observed increase in the crystalline orientation and decrease in permeability. Fibers annealed above 190 degrees C did not show the exotherm and had significantly fewer microcrystals. The crystallization in this regime was attributed to the growth of existing lamellae, as evidenced by the increase in crystallite size, crystalline density, crystalline orientation, lamellar spacing, and lamellar intensity. The changes at annealing temperatures > 190 degrees C are accompanied by increased dyeability, indicative of more open amorphous regions. (c) 2006 Wiley Periodicals, Inc.