Nylon 6 and 66 yarns were heatset at different temperatures in a Suessen unit; near infra-red (NIRA) and x-ray diffraction analyses were then made on these samples. A regression modeled calibration curve was established by measuring the NIRA reflectance of yarns heatset at known temperatures. This curve was, in turn, used to determine effectively the heatset temperature of nylon 6 and 66 yarns. Correlation of the NIRA data with x-ray diffraction data indicates that the NIRA regression models detect and measure crystalline perfection and growth with the increase in Suessen heatset temperature. -- AATA