Polarized IR spectra of alternating copolymers and random ones of butadiene with acrylonitrile have been investigated in the region from 4000 to 250 cm−1. The spectra of deuterated alternating copolymers have also been measured. It has been found that dichroism appears to a marked extent when films of the alternating copolymer are elongated, whereas it is hardly observable for the random one. The results were discussed from a viewpoint of orientation of polymer molecules and crystallites in the films of alternating copolymer. Tentative assignments of the absorption bands were made with reference to the results for trans-1,4-polybutadiene and those for polyacrylonitrile. The molecular structure of the alternating copolymer is discussed.