The origins of characteristic bands in the infrared spectrum of isotactic polystyrene have been studied with the aid of the spectrum of isotactic poly (ring‐d5styrene). It is found that these bands can be classified into three types according to their experimental behavior upon crystallization of the specimen. Assignments show that the bands in any one group are associated with a specific kind of normal vibration, either of the chain or of the benzene ring. Analysis of one of these band types indicates that rotational disordering of the benzene rings may occur, and that crystallization of the polymer is probably correlated with the growth of regions which maintain statistical variability at the local chain level.