Ab initio SCF-MO calculations, mainly at the 6-31G F* level with 3-21G F* fully optimized geometries, were performed for 2- and 3-fluorostyrene in different comformations. Structures and conformational preferences of these molecules are compared with available data and discussed. It was found that the 2-fluorostyrene molecule has acis-trans energy difference of ca. 2.4 kJ mol¯¹, showing a small barrier to planarity of 0.26 kJ mol¯¹ in thetrans form. While the reduced stability of thecis form is mainly ascribed to a repulsive F … vinyl group interaction, the partial loss of resonance stabilization between the ring and the vinyl group in the trans form and the H6 … Hc repulsive interaction in this form are taken to explain the local maximum atθ=180‡. In the 3-fluorostyrene molecule, thecis form is favored over thetrans by ca. 0.5 kJ mol−1. An appreciable asymmetry of charge distribution found for thetrans form is assumed to slightly overweigh the difference in nuclear repulsion energy favorable to this conformation, thus explaining the observed relative stability.