The morphology, mechanical properties, and dye diffusion of drawn and heat-set polyamide-6 (PA6) yarns were examined. Correlations between the microstructure of PA6 yarns and the dye diffusion coefficients and mechanical properties were established. The crystallinity of PA6 yarns was estimated with density and Fourier transform infrared spectroscopy measurements. A decrease in the T crystallinity and an increase in the gamma-crystallite size with the draw ratio were observed and attributed to the disappearance of small crystallites and an increase in the average gamma-crystallite size population during the deformation process. The scouring treatment increased the total crystallinity, almost entirely as a result of an increase in the a fraction. Thermally induced crystallization involved increases in both crystalline phases (alpha and gamma) and did not involve crystal-to-crystal transformation, whereas drawing PA6 yarns involved both crystallization of the amorphous phase in the alpha form and gamma ->alpha transformation. A sharp decrease in the diffusion coefficient with an increasing draw ratio of PA6 yarns was correlated with an increasing amorphous orientation. The influence of thermally induced crystallinity on the diffusion coefficient seemed exceptionally strong. The mechanical properties of PA6 yarns were examined and correlated with structural changes. It was demonstrated that the crystallinity had a direct correlation with the terminal modulus and extension at break, whereas there was no correlation with the initial modulus. (c) 2006 Wiley Periodicals, Inc.