The i.r. spectra of the liquid and crystalline phases of fumaryl chloride were recorded from 4000 to 100 cm −1. The Raman spectrum of liquid fumaryl chloride was recorded from 2500 to 150 cm −1. Comparison of the liquid and solid phase i.r. spectra indicates the presence of two planar conformers in an equilibrium mixture in the liquid phase: trans-trans with a C 2h structure, and cis-trans with a C s structure. The spectrum of the crystal consists solely of the trans-trans conformer. A vibrational assignment of the trans-trans conformer has been made. A partial assignment of the cis-trans conformer has been proposed. Oriented polycrystalline films of fumaryl chloride were grown on alkali halide and polyethylene windows at sub-ambient temperatures. The polarized spectra of these films were studied at about 30 K for the films grown on alkali halide windows and at liquid N 2 temperatures for those films grown on polyethylene windows. The results indicate that the trans-trans conformer occupies a C i site in the unit cell. The factor group splitting and polarization evidenced are consistent with a factor group of D 2 h for an orthorhombic crystal with four molecules/Bravais cell.