Polysilaethylene ([SiH2CH2](n)) (PSE) and deuterated polysilaethylene ([SiD2CH2](n)) have been examined by both vibrational spectroscopy and X-ray diffraction methods above and below the crystallization/melting transition at -20 degrees C, The results confirm previous indications that PSE crystallizes in an all-trans conformation analogous to that found for its all-carbon analog, polyethylene. Raman spectroscopy reveals the appearance of a sharp band at 706 cm(-1) attributable to the symmetric Si-C stretching vibration when PSE is cooled below 0 degrees C, while wide-angle X-ray diffraction shows the existence of a crystalline form with a monoclinic unit cell (alpha = 5.70 Angstrom, b = 8.75 Angstrom, c = 3.25 Angstrom; gamma = 97.5 degrees) whose dimensions compare closely to the calculated repeat distance using the planar zigzag conformation of PSE. The calculation of the IR spectra by ab initio methods (Hartree-Fock level of theory) using the 3-21G* and 6-31G" basis sets agrees well with the experimental IR results. Through the use of the calculated IR spectra, symmetry analysis, and spectroscopic data on the deuterated sample, the assignments of the different PSE vibrational bands have been achieved.