The infrared spectra of poly(acetylene), poly(acetylene-d2), copoly(acety+acetylene-d2), and copoly(acetylene+acetylene-d1+acetylene-d2) prepared by the Ti(OC4H9)4–Al(C2H5)3 system over a wide temperature range (ca. −100 to 180°C) are reported. A tentative assignment of the observed spectra is made on the basis of model structures in which infinite planar chains of all trans, all trans–cisoid, and all cis–transoid configurations are assumed. The spectral data are best interpreted on the basis of an all cis-transoid (or an all trans-cisoid) structure for the polymers prepared at temperatures lower than −78°C, and an all trans structure for the polymers prepared at temperatures higher than 150°C.Simplified calculations of the C-H and C-D out-of-plane deformation frequencies are made for various model chains. It has been concluded from a comparison of the observed and calculated frequencies that the cis-opening of the triple bond occurs in a polymerization reaction with the Ti(OC4H9)4–Al(C2H5)3 catalyst system at low temperatures.