Infra-red spectra of polypropylene oxide, polybutadiene monoxide and polystyrene oxide have been studied. 1.(1) Polypropylene oxideDetailed assignments of the infra-red absorptions of this polymer in the region of 3300–650 cm−1 have been made with the help of polarized spectra of oriented samples. These assignments suggest that the major differences between the infra-red spectra of amorphous and crystalline samples of polypropylene oxide are due to rotational isomerism. It is proposed that trans and gauche forms exist in the amorphous portion of the polymer and that the trans form exists in the crystalline portion of the polymer.2.(2) Polybutadiene monoxideThe bands assigned to the open chain C-O-C stretching mode and the two bands assigned to the CH2 and CH out-of-plane deformation modes of the vinyl group were observed and it is suggested that the polymer results from successive opening of the epoxy-ring. Polarized infra-red spectra suggest that the polymer possesses a compressed trans zigzag configuration.3.(3) Polystyrene oxideThe infra-red dichroism of the 697 and 755 cm−1 bands is perpendicular, and consequently the plane of the benzene ring may be nearly parallel to the direction of orientation. The shape and intensity of some bands are similar to those of crystalline isotactic polystyrene. Consequently the molecular structure of polystyrene oxide may be helical.