The infrared spectra of 3,3,3-trifluoropropene and hexafluoropropene were studied in the vapour phase and in solution between 4000 and 40 cm−1. Raman spectra of the compounds as vapours and liquids were recorded and polarization data obtained. The spectra of both molecules are interpreted in terms of Cs symmetry. Complete vibrational assignments, including the torsional mode, are presented, based upon infrared and Raman vapour contours, Raman polarization measurements and the results of a normal coordinate analysis. © 1979, Walter de Gruyter. All rights reserved.