For the halogenated (X = F, Cl, Br) propenes X 2CCXCX 3, X 2CCHCX 3, X 2CCXCHX 2, trans-XHCCXCX 3, X 2CCXCH 2X, X 2CCHCHX 2 and trans-XHCCXCHX 2 torsional potentials and conformational structures are obtained using molecular mechanics calculations. It has been predicted that for some of these molecules the low-energy conformation is not the one suggested by considering molecular models. Thus, for the molecules X 2CCXCX 3 (X = Cl, Br) one of the X atoms in the CX 3 group is coplanar with the atoms X 2CCXC and has an anti position relative to the CC bond, while for the molecule F 2CCFCF 3 one of the F atoms in the CF 3 group has a syn position relative to the CC bond. The chloro and bromo substituted molecules may also exist as mixtures of anti and syn conformers. Only in the compounds X 2CCXCH 2X is gauche the stable form for XCl and Br. Syn is a stable form for all the fluoro substituted molecules studied here, but anti is never a stable form for these molecules. Except for the molecules possessing a CF 3 group, gauche is a stable conformation in the fluoro substituted molecules. Gauche is the only stable conformation for the molecules X 2CCXCH 2X with X = Cl or Br. Values of the torsional barrier heights have been obtained between 0.2 and 14 kcal mol −1. However, for the fluoro substituted molecules the barrier heights are limited to the range 0.2–3.5 kcal mol −1. Values of torsional force constants and structural parameters are recorded for all molecules. Also the conformational possibilities for the 21 molecules cis-XHCCXCX 3, cis-XHCCHCX 3, H 2CCXCX 3, cis-XHCCXCHX 2, cis-XHCCXCH 2X, cis-XHCCHCHX 2 and H 2CCXCHX 2 (X = F, Cl, Br) have been estimated.