Having found much ambiguity in the infrared band assignments for polyamide 66 (PA66), we revisited some of these assignments before using infrared spectroscopy to assess microstructure changes resulting from multiple thermal treatments. We discovered that earlier assignments of the 1144 and 1180 cm(-1) bands to the amorphous (noncrystalline) phase were incorrect, whereas the bands at 924 and 1136 cm(-1) can be attributed unambiguously to the noncrystalline phase. We also confirmed cm that PA66 bands at 936 and 1200 cm(-1) are crystalline bands. The normalized absorbance of the 1224-cm(-1) fold band increases in proportion to crystallinity, indicating that chain folding is the predominant mechanism of thermal crystallization in PA66. We demonstrated that infrared spectroscopy can be used to estimate the degree of crystallinity of PA66, and two methods were explored. One is a calibration method in which the band ratio of 1200 and 1630 cm(-1) is plotted against crystallinity measured by density. The other is an independent infrared method based on the assumption that PA66 satisfies a two-phase structure model. The crystallinity determined by the independent infrared method showed good agreement with the crystallinity obtained from density measurements.