Following a study of the liquid phase of the title molecule by NMR and u.v. methods, in which the angle between the olefin and ring planes was found to be τ $ ̃ 70°, a gas-phase electron diffraction study has been carried put. The structural analysis was based on diffraction data for which the effect of the perfluorophenyl group had been subtracted out. Prior to the optimization of vibrational amplitudes, two minima were found in least-squares refinements: I with τ $ ̃ 90δ and e $ ̃ 13° and II with τ $ ̃ 106° and ϵ $ ̃ 37° (ϵ is the angle of rotation around the CF 3-C bond; ϵ = 0° corresponds to eclipsing of a C-F bond with the olefin double bond). After the amplitudes had been fitted to a simple model relating them to the torsional motion around the CF 3-C and C 6F 3-C bonds, minimum I disappeared, to be replaced by in in which τ = 90° and ϵ = 0°. Minimum III gives a preferred conformation for the molecule having symmetry C 5, higher symmetry than found in the liquid phase studies. The presence of a second conformer cannot be ruled out-entirely.