Our previous work showed that drawing polyamide 66 (PA 66) fibers at room temperature does not change the degree of crystallinity, but only increases the molecular orientation. We therefore have used a series PA 66 fibers with different draw ratios to establish a direct correlation between (noncrystalline) molecular orientation f(a) and the dye diffusion coefficient D. For both acid and disperse dyes, the relationship log D proportional to f(a)(2`) provides reasonable fits to the data for PA 66 fibers, and a similar relationship appears to be applicable to PA 6 fibers. Heat-setting the fibers results in a continuous decrease in diffusion coefficient; unlike PA6 and PET fibers, no minimum in D was found in the region of 160degreesC. If this decrease in D is attributed to the increase in volume fraction crystallinity X taking place during heat-setting, it must be deduced that log D proportional to X-6. This dependence is surprisingly strong, but is consistent with observations we have made on PET and PA6 fibers. It is possible that some other structural rearrangement is partially or largely responsible for the decrease in diffusion coefficient, but Fourier transform infrared, density, and X-ray diffraction measurements do not indicate any other structural changes taking place. (C) 2003 Wiley Periodicals, Inc.