The infrared spectrum of polyacrylonitrile has been obtained in the extended region of 70 to 3200 cm.−1 Polarized measurements were made on stretched and rolled specimens in the range of 350 to 3200 cm.−1. An analysis of the spectrum has been made on the basis of an atactic chain configuration, and has been shown to be in agreement with the observed data. Detailed assignments are proposed for all of the absorption bands in the spectrum, being aided by previous studies of high polymer spectra and by approximate normal frequency calculations for the nitrile group.